??xml version="1.0" encoding="utf-8" standalone="yes"?>男人的天堂亚洲一区二区三区,浮力影院亚洲国产第一页,亚洲人成电影网站色wwwhttp://www.tkk7.com/sharajava/zh-cnSun, 11 May 2025 17:49:19 GMTSun, 11 May 2025 17:49:19 GMT60cȝ动态加?/title><link>http://www.tkk7.com/sharajava/archive/2006/07/27/60278.html</link><dc:creator>sharajava</dc:creator><author>sharajava</author><pubDate>Thu, 27 Jul 2006 01:06:00 GMT</pubDate><guid>http://www.tkk7.com/sharajava/archive/2006/07/27/60278.html</guid><wfw:comment>http://www.tkk7.com/sharajava/comments/60278.html</wfw:comment><comments>http://www.tkk7.com/sharajava/archive/2006/07/27/60278.html#Feedback</comments><slash:comments>0</slash:comments><wfw:commentRss>http://www.tkk7.com/sharajava/comments/commentRss/60278.html</wfw:commentRss><trackback:ping>http://www.tkk7.com/sharajava/services/trackbacks/60278.html</trackback:ping><description><![CDATA[ <h3 style="MARGIN: 13pt 0cm"> <font size="5"> <span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">?/span> <span lang="EN-US">: </span> <span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">调用</span> <span lang="EN-US">Class.forName()</span> <span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">?/span> <span lang="EN-US">ClassLoader.loadClass()</span> <span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的区别在什么地?/span> <span lang="EN-US">?</span> </font> </h3> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; LINE-HEIGHT: 150%"> <span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">{?/span> <span lang="EN-US">: </span> <span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">q两Ҏ(gu)都是通过一个给定的cdd位和加蝲q个cd对应?/span> <span lang="EN-US">java.long.Class</span> <span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">cd?/span> <span lang="EN-US">. </span> <span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">管如此</span> <span lang="EN-US">, </span> <span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">它们的在行ؓ(f)方式上还是有区别?/span> <span lang="EN-US">. </span> </p> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt 42pt; TEXT-INDENT: -21pt; LINE-HEIGHT: 150%; mso-list: l0 level1 lfo1; tab-stops: list 42.0pt"> <span lang="EN-US" style="FONT-FAMILY: Wingdings; mso-bidi-font-family: Wingdings; mso-fareast-font-family: Wingdings"> <span style="mso-list: Ignore">Ø<span style="FONT: 7pt 'Times New Roman'">         </span></span> </span> <span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">用哪?/span> <span lang="EN-US">java.lang.ClassLoader</span> <span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">q行加蝲</span> <span lang="EN-US" style="FONT-FAMILY: 宋体"> <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /?> <o:p> </o:p> </span> </p> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt 42pt; TEXT-INDENT: -21pt; LINE-HEIGHT: 150%; mso-list: l0 level1 lfo1; tab-stops: list 42.0pt"> <span lang="EN-US" style="FONT-FAMILY: Wingdings; mso-bidi-font-family: Wingdings; mso-fareast-font-family: Wingdings"> <span style="mso-list: Ignore">Ø<span style="FONT: 7pt 'Times New Roman'">         </span></span> </span> <span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">q回?/span> <span lang="EN-US">Class</span> <span style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">对象是否被初始化</span> <span lang="EN-US" style="FONT-FAMILY: 宋体"> <o:p> </o:p> </span> </p> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; LINE-HEIGHT: 150%"> <span lang="EN-US" style="FONT-FAMILY: 宋体">Class.forName(String)</span> <span style="FONT-FAMILY: 宋体">Ҏ(gu)<span lang="EN-US">(</span>只有一个参?span lang="EN-US">), </span>使用调用者的cd载器来加?span lang="EN-US">, </span>也就是用加蝲了调?span lang="EN-US">forName</span>Ҏ(gu)的代码的那个cd载器<span lang="EN-US">. </span>相应?span lang="EN-US">, ClassLoader.loadClass()</span>Ҏ(gu)是一个实例方?span lang="EN-US">(</span>非静态方?span lang="EN-US">), </span>调用旉要自己指定类加蝲?span lang="EN-US">, </span>那么q个cd载器可能是也可能不是加载调用代码的cd载器<span lang="EN-US">. </span>如果用特定的cd载器来加载类在你的设计中占有比较重要的地?span lang="EN-US">, </span>你就应该调用<span lang="EN-US">ClassLoader.loadClass(String)</span>Ҏ(gu)?span lang="EN-US">Class.forName(String, boolean, ClassLoader)</span>Ҏ(gu)<span lang="EN-US">. <o:p></o:p></span></span> </p> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; LINE-HEIGHT: 150%"> <span lang="EN-US" style="FONT-FAMILY: 宋体"> <span style="mso-tab-count: 1">    </span> </span> <span style="FONT-FAMILY: 宋体">另外<span lang="EN-US">, Class.forName()</span>Ҏ(gu)对加载的cd象进行初始化<span lang="EN-US">. </span>可见的效果就是类中静态初始化D及(qing)字节码中Ҏ(gu)有静态成员的初始工作的执?span lang="EN-US">(</span>q个q程在类的所有父cM递归地调?span lang="EN-US">). </span>q点׃<span lang="EN-US">ClassLoader.loadClass()</span>不同<span lang="EN-US">. ClassLoader.loadClass()</span>加蝲的类对象是在W一ơ被调用时才q行初始化的<span lang="EN-US">. <o:p></o:p></span></span> </p> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; LINE-HEIGHT: 150%"> <span lang="EN-US" style="FONT-FAMILY: 宋体"> <span style="mso-tab-count: 1">    </span> </span> <span style="FONT-FAMILY: 宋体">你可以利用上q的差异<span lang="EN-US">. </span>比如<span lang="EN-US">,</span>要加载一个静态初始化开销很大的类<span lang="EN-US">, </span>你就可以选择提前加蝲该类<span lang="EN-US">(</span>以确保它?span lang="EN-US">classpath</span>?span lang="EN-US">), </span>但不q行初始?span lang="EN-US">, </span>直到W一ơ用该cȝ域或Ҏ(gu)时才q行初始?span lang="EN-US">. <o:p></o:p></span></span> </p> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; LINE-HEIGHT: 150%"> <span lang="EN-US" style="FONT-FAMILY: 宋体"> <span style="mso-tab-count: 1">    </span> </span> <span style="FONT-FAMILY: 宋体">最常用的是<span lang="EN-US">Class.forName(String, boolean, ClassLoader). </span>讄W二个参Cؓ(f)<span lang="EN-US">false</span>xq初始化<span lang="EN-US">, </span>W三个参数指定要用来q行加蝲的类加蝲?span lang="EN-US">. </span>我徏议ؓ(f)了最大的灉|性用这个方?span lang="EN-US">. <o:p></o:p></span></span> </p> <h2 style="MARGIN: 13pt 0cm"> <span style="FONT-FAMILY: 黑体; mso-ascii-font-family: Arial">cd始化错误是难处理?/span> </h2> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; LINE-HEIGHT: 150%"> <span lang="EN-US" style="FONT-FAMILY: 宋体"> <span style="mso-tab-count: 1">    </span> </span> <span style="FONT-FAMILY: 宋体">成功地加载了c?span lang="EN-US">, </span>q不意味着׃?x)有其它问?span lang="EN-US">. </span>静态初始化代码可以抛出异常<span lang="EN-US">, </span>异常被包装到<span lang="EN-US">java.long.ExceptionInInitializerError</span>的实例中<span lang="EN-US">. </span>异常抛出?span lang="EN-US">, </span>q个cd不可?span lang="EN-US">. </span>q样<span lang="EN-US">, </span>如果你需要在代码中处理这些错?span lang="EN-US">, </span>你就应该调用q行初始化的<span lang="EN-US">Class.forName()</span>Ҏ(gu)<span lang="EN-US">. <o:p></o:p></span></span> </p> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; LINE-HEIGHT: 150%"> <span lang="EN-US" style="FONT-FAMILY: 宋体"> <span style="mso-tab-count: 1">    </span> </span> <span style="FONT-FAMILY: 宋体">但进一步说<span lang="EN-US">, </span>如果你要处理<span lang="EN-US">ExceptionInInitializerError</span>q试图从错误中恢?span lang="EN-US">, </span>很可能不如你惌的那h常工?span lang="EN-US">. </span>L(fng)下面的示例代?span lang="EN-US">:<o:p></o:p></span></span> </p> <p class="MsoNormal" style="BACKGROUND: #eeeeee; MARGIN: 0cm 0cm 12pt; WORD-BREAK: break-all; TEXT-ALIGN: left; mso-pagination: widow-orphan" align="left"> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> <br /> </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">public</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">class</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> Main<br />{<br />    </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">public</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">static</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">void</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> main (String [] args) </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">throws</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> Exception<br />    {<br />        </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">for</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> (</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">int</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> repeat = 0; repeat < 3; ++ repeat)<br />        {<br />            </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">try</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> <br />            {<br />                </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: green; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">// "Real" name for X is outer class name+$+nested class name:<br /></span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">                Class.forName ("Main$X");<br />            }<br />            </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">catch</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> (Throwable t)<br />            {<br />                System.out.println ("load attempt #" + repeat + ":");<br />                t.printStackTrace (System.out);<br />            }<br />        }<br />    }<br /><br />    </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">private</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">static</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">class</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> X<br />    {<br />        </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">static</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> <br />        {<br />            </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">if</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> (++ s_count == 1)<br />                </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">throw</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">new</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> RuntimeException ("failing static initializer<?xml:namespace prefix = v ns = "urn:schemas-microsoft-com:vml" /?><v:shapetype id="_x0000_t75" stroked="f" filled="f" path="m@4@5l@4@11@9@11@9@5xe" o:preferrelative="t" o:spt="75" coordsize="21600,21600"><v:stroke joinstyle="miter"></v:stroke><v:formulas><v:f eqn="if lineDrawn pixelLineWidth 0"></v:f><v:f eqn="sum @0 1 0"></v:f><v:f eqn="sum 0 0 @1"></v:f><v:f eqn="prod @2 1 2"></v:f><v:f eqn="prod @3 21600 pixelWidth"></v:f><v:f eqn="prod @3 21600 pixelHeight"></v:f><v:f eqn="sum @0 0 1"></v:f><v:f eqn="prod @6 1 2"></v:f><v:f eqn="prod @7 21600 pixelWidth"></v:f><v:f eqn="sum @8 21600 0"></v:f><v:f eqn="prod @7 21600 pixelHeight"></v:f><v:f eqn="sum @10 21600 0"></v:f></v:formulas><v:path o:connecttype="rect" gradientshapeok="t" o:extrusionok="f"></v:path><o:lock aspectratio="t" v:ext="edit"></o:lock></v:shapetype><v:shape id="_x0000_i1025" style="WIDTH: 11.25pt; HEIGHT: 15pt" alt="" type="#_x0000_t75"><v:imagedata o:href="/Images/dot.gif" src="file:///C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\msohtml1\01\clip_image001.gif"></v:imagedata></v:shape>");<br />        }<br />        <br />    } </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: green; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">// End of nested class<br /></span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> <br />    </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">private</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">static</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">int</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> s_count;<br /><br />} </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: green; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">// End of class<br style="mso-special-character: line-break" /><br style="mso-special-character: line-break" /></span> <span lang="EN-US" style="FONT-SIZE: 10pt; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> <o:p> </o:p> </span> </p> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; LINE-HEIGHT: 150%"> <span lang="EN-US" style="FONT-FAMILY: 宋体"> <span style="mso-tab-count: 1">    </span> </span> <span style="FONT-FAMILY: 宋体">上面的代?span lang="EN-US">3</span>ơ尝试加载一个内部类<span lang="EN-US">X, </span>即便?span lang="EN-US">X</span>的静态初始化只在每一ơ加载时p|<span lang="EN-US">, </span>q?span lang="EN-US">3</span>ơ加载都抛出了异?span lang="EN-US">. <o:p></o:p></span></span> </p> <p> <font face="宋体"> <span lang="EN-US" style="FONT-SIZE: 11.5pt; BACKGROUND: #d9d9d9; mso-shading: white; mso-pattern: gray-15 auto">>java Main<br />load attempt #0:<br />java.lang.ExceptionInInitializerError<br />        at java.lang.Class.forName0(Native Method)<br />        at java.lang.Class.forName(Class.java:140)<br />        at Main.main(Main.java:17)<br />Caused by: java.lang.RuntimeException: failing static initializer...<br />        at Main$X.<clinit>(Main.java:40)<br />        ... 3 more<br />load attempt #1:<br />java.lang.NoClassDefFoundError<br />        at java.lang.Class.forName0(Native Method)<br />        at java.lang.Class.forName(Class.java:140)<br />        at Main.main(Main.java:17)<br />load attempt #2:<br />java.lang.NoClassDefFoundError<br />        at java.lang.Class.forName0(Native Method)<br />        at java.lang.Class.forName(Class.java:140)<br />        at Main.main(Main.java:17)</span> <span lang="EN-US" style="BACKGROUND: #d9d9d9; mso-shading: white; mso-pattern: gray-15 auto"> <o:p> </o:p> </span> </font> </p> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; LINE-HEIGHT: 150%"> <span lang="EN-US" style="FONT-FAMILY: 宋体"> <span style="mso-tab-count: 1">    </span> </span> <span style="FONT-FAMILY: 宋体">有点令h吃惊的时<span lang="EN-US">, </span>在第<span lang="EN-US">2, 3</span>ơ进行类加蝲?span lang="EN-US">, </span>抛出的异常竟然是<span lang="EN-US">java.lang.NoClassDefFoundError. </span>q里发生的事情是<span lang="EN-US">, </span>W一ơ加载后<span lang="EN-US">(</span>在进行初始化之前<span lang="EN-US">), JVM</span>发现<span lang="EN-US">X</span>已经被加?span lang="EN-US">, </span>而这?span lang="EN-US">X</span>的类实例在加载它的类加蝲器被垃圾回收之前是不?x)被卸蝲?span lang="EN-US">. </span>所以这之后的对<span lang="EN-US">Class.forName()</span>的调用时<span lang="EN-US">, JVM</span>不会(x)再尝试进行初始化的工?span lang="EN-US">, </span>但是<span lang="EN-US">, </span>更o(h)Z解的?span lang="EN-US">, </span>抛出一?span lang="EN-US">NoClassDefFoundError. <o:p></o:p></span></span> </p> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; LINE-HEIGHT: 150%"> <span lang="EN-US" style="FONT-FAMILY: 宋体"> <span style="mso-tab-count: 1">    </span> </span> <span style="FONT-FAMILY: 宋体">卸蝲q样的类的方法是丢弃原来加蝲该类的类加蝲器实例ƈ重新创徏一?span lang="EN-US">. </span>当然<span lang="EN-US">, </span>q只能是在你使用?span lang="EN-US">Class.forName(String, boolean, ClassLoader)</span>q个<span lang="EN-US">3</span>参数的方法的时候才能办?span lang="EN-US">.<o:p></o:p></span></span> </p> <h2 style="MARGIN: 13pt 0cm"> <span style="FONT-FAMILY: 黑体; mso-ascii-font-family: Arial">隐藏?/span> <span lang="EN-US"> <font face="Arial">Class.forName()</font> </span> <span style="FONT-FAMILY: 黑体; mso-ascii-font-family: Arial">Ҏ(gu)</span> </h2> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; LINE-HEIGHT: 150%"> <span lang="EN-US" style="FONT-FAMILY: 宋体"> <span style="mso-tab-count: 1">    </span> </span> <span style="FONT-FAMILY: 宋体">你一定用q?span lang="EN-US">Java</span>?span lang="EN-US">X.class</span>的语法去获取一个在~译器就知道名字的类对象实例<span lang="EN-US">. </span>在字节码的层ơ上<span lang="EN-US">, </span>q一Ҏ(gu)如何实现的就不被人熟知了<span lang="EN-US">. </span>不同的编译器有不同的实例l节<span lang="EN-US">, </span>但共同点?span lang="EN-US">, </span>所有编译器所相应产生的代码都是调用的<span lang="EN-US">Class.forName(String)</span>q一个参数的Ҏ(gu)<span lang="EN-US">. </span>比如<span lang="EN-US">J2SE <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" /?><st1:chsdate w:st="on" year="1899" month="12" day="30" islunardate="False" isrocdate="False">1.4.1</st1:chsdate></span>?span lang="EN-US">javac</span>把<span lang="EN-US">Class cls = X.class; </span>译成如下等L(fng)形式<span lang="EN-US">:<o:p></o:p></span></span> </p> <p class="MsoNormal" style="BACKGROUND: #eeeeee; MARGIN: 0cm 0cm 12pt; WORD-BREAK: break-all; TEXT-ALIGN: left; mso-pagination: widow-orphan" align="left"> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> <v:shape id="_x0000_i1026" style="WIDTH: 11.25pt; HEIGHT: 15pt" alt="" type="#_x0000_t75"><v:imagedata o:href="/Images/dot.gif" src="file:///C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\msohtml1\01\clip_image001.gif"></v:imagedata></v:shape><br />        </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: green; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">// This is how "Class cls = X.class" is transformed:<br /></span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">        </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">if</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> (</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">class</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">$Main$X == </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">null</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">)<br />        {<br />            </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">class</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">$Main$X = </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">class</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">$ ("Main$X");<br />        }<br />        Class cls = </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">class</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">$Main$X;<br /><br />    <v:shape id="_x0000_i1027" style="WIDTH: 11.25pt; HEIGHT: 15pt" alt="" type="#_x0000_t75"><v:imagedata o:href="/Images/dot.gif" src="file:///C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\msohtml1\01\clip_image001.gif"></v:imagedata></v:shape><br /><br />    </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">static</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> Class </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">class</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">$ (String s)<br />    {<br />        </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">try</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> <br />        {<br />            </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">return</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> Class.forName (s);<br />        }<br />        </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">catch</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> (ClassNotFoundException e)<br />        {<br />            </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">throw</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">new</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> NoClassDefFoundError (e.getMessage());<br />        }<br />    }<br /><br />    </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">static</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> Class </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">class</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">$Main$X; </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: green; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">// A synthetic field created by the compiler</span> <span lang="EN-US" style="FONT-SIZE: 10pt; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> <o:p> </o:p> </span> </p> <h2 style="MARGIN: 13pt 0cm"> <span style="FONT-FAMILY: 黑体; mso-ascii-font-family: Arial">?/span> <span lang="EN-US"> <font face="Arial">Sun</font> </span> <span style="FONT-FAMILY: 黑体; mso-ascii-font-family: Arial">?/span> <span lang="EN-US"> <font face="Arial">javac</font> </span> <span style="FONT-FAMILY: 黑体; mso-ascii-font-family: Arial">开个玩W?/span> </h2> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 21pt; LINE-HEIGHT: 150%"> <span style="FONT-FAMILY: 宋体">从上面的例子你可以看?span lang="EN-US">, </span>~译器调?span lang="EN-US">Class.forName()</span>Ҏ(gu)加蝲cd?span lang="EN-US">, </span>q将其缓存到一个包内可见的静态变量中<span lang="EN-US">. </span>q种令h费解的实现方式的可能是因为在早期版本?span lang="EN-US">Java</span>?span lang="EN-US">, </span>q种<span lang="EN-US">X.class</span>的语法还未被支持<span lang="EN-US">, <span style="COLOR: #ffcc00">so the feature was added on top of the Java 1.0 byte-code instruction set</span>.(???)<o:p></o:p></span></span> </p> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 21pt; LINE-HEIGHT: 150%"> <span style="FONT-FAMILY: 宋体">利用q一?span lang="EN-US">, </span>你可以在~译器的开销上做一些有的事情<span lang="EN-US">. </span>?span lang="EN-US">J2SE <st1:chsdate w:st="on" year="1899" month="12" day="30" islunardate="False" isrocdate="False">1.3.1</st1:chsdate></span>~译下面的代码片D?span lang="EN-US">: <o:p></o:p></span></span> </p> <p class="MsoNormal" style="BACKGROUND: #eeeeee; MARGIN: 0cm 0cm 0pt; WORD-BREAK: break-all; TEXT-ALIGN: left; mso-pagination: widow-orphan" align="left"> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">public</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">class</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> <st1:place w:st="on">Main</st1:place><br />{<br />    </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">public</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">static</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">void</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> main (String [] args) </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">throws</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> Exception<br />    {<br />        System.out.println ("String class: " + String.</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">class</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">);<br />        </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">class</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">$java$lang$String = </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">int</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">.</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">class</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">;<br />        System.out.println ("String class: " + String.</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">class</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">);<br />    }<br />    <br />    </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">static</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> Class </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">class</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">$java$lang$String;<br /><br />} </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: green; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">// End of class</span> <span lang="EN-US" style="FONT-SIZE: 10pt; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> <o:p> </o:p> </span> </p> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; LINE-HEIGHT: 150%"> <span style="FONT-FAMILY: 宋体">q行?span lang="EN-US">, </span>你会(x)得到下面q个很荒谬的输出l果<span lang="EN-US">: <o:p></o:p></span></span> </p> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt"> <span lang="EN-US" style="BACKGROUND: #d9d9d9; FONT-FAMILY: 宋体; mso-shading: white; mso-pattern: gray-15 auto">>java <st1:place w:st="on">Main</st1:place><o:p></o:p></span> </p> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 5.25pt; mso-char-indent-count: .5"> <span lang="EN-US" style="BACKGROUND: #d9d9d9; FONT-FAMILY: 宋体; mso-shading: white; mso-pattern: gray-15 auto">String class: class java.lang.String<o:p></o:p></span> </p> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 5.25pt; mso-char-indent-count: .5"> <span lang="EN-US" style="BACKGROUND: #d9d9d9; FONT-FAMILY: 宋体; mso-shading: white; mso-pattern: gray-15 auto">String class: int<o:p></o:p></span> </p> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; LINE-HEIGHT: 150%"> <span style="FONT-FAMILY: 宋体">?span lang="EN-US">J2SE <st1:chsdate w:st="on" year="1899" month="12" day="30" islunardate="False" isrocdate="False">1.4.1</st1:chsdate></span>?span lang="EN-US">, </span>上面的代码将不能被编译通过<span lang="EN-US">, </span>但你仍然可以用反的方式戏弄?span lang="EN-US">: <o:p></o:p></span></span> </p> <p class="MsoNormal" style="BACKGROUND: #eeeeee; MARGIN: 0cm 0cm 0pt; WORD-BREAK: break-all; TEXT-ALIGN: left; mso-pagination: widow-orphan" align="left"> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">public</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">static</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">void</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> main (String [] args) </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">throws</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> Exception<br />    {<br />        System.out.println ("String class: " + String.</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">class</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">);<br />        Main.</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">class</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">.getDeclaredField ("class$java$lang$String").set (</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">null</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">, </span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">int</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">.</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">class</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">);<br />        System.out.println ("String class: " + String.</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: blue; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">class</span> <span lang="EN-US" style="FONT-SIZE: 10pt; COLOR: black; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt">);<br />    }</span> <span lang="EN-US" style="FONT-SIZE: 10pt; FONT-FAMILY: 宋体; mso-bidi-font-family: 宋体; mso-font-kerning: 0pt"> <o:p> </o:p> </span> </p> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; LINE-HEIGHT: 150%"> <span lang="EN-US" style="FONT-FAMILY: 宋体"> <o:p> </o:p> </span> </p> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; LINE-HEIGHT: 150%"> <span lang="EN-US" style="FONT-FAMILY: 宋体"> <span style="mso-tab-count: 1">    </span> </span> <span style="FONT-FAMILY: 宋体">lg所q?span lang="EN-US">, </span>下次你再调用<span lang="EN-US">Class.forName()</span>Ҏ(gu)?span lang="EN-US">, </span>你应该知道它的局限性可选的替代Ҏ(gu)?span lang="EN-US">. <o:p></o:p></span></span> </p> <p class="MsoNormal" style="MARGIN: 0cm 0cm 0pt; LINE-HEIGHT: 150%"> <span lang="EN-US" style="FONT-FAMILY: 宋体"> <o:p> </o:p> </span> </p> <p> </p> <img src ="http://www.tkk7.com/sharajava/aggbug/60278.html" width = "1" height = "1" /><br><br><div align=right><a style="text-decoration:none;" href="http://www.tkk7.com/sharajava/" target="_blank">sharajava</a> 2006-07-27 09:06 <a href="http://www.tkk7.com/sharajava/archive/2006/07/27/60278.html#Feedback" target="_blank" style="text-decoration:none;">发表评论</a></div>]]></description></item><item><title>JBoss MBean服务加蝲q程http://www.tkk7.com/sharajava/archive/2006/07/25/59952.htmlsharajavasharajavaTue, 25 Jul 2006 03:18:00 GMThttp://www.tkk7.com/sharajava/archive/2006/07/25/59952.htmlhttp://www.tkk7.com/sharajava/comments/59952.htmlhttp://www.tkk7.com/sharajava/archive/2006/07/25/59952.html#Feedback0http://www.tkk7.com/sharajava/comments/commentRss/59952.htmlhttp://www.tkk7.com/sharajava/services/trackbacks/59952.htmlhttp://www.tkk7.com/images/blogjava_net/sharajava/13416/o_start-jboss-mbean-service.JPG 

 

1.      前缀1.1的方?span lang="EN-US">, 代表加蝲和解?span lang="EN-US">XML服务描述文g的过E?span lang="EN-US">.

2.      前缀1.2的方?span lang="EN-US">, 代表?span lang="EN-US">XML服务描述文g中类路径条目的处?span lang="EN-US">. q个q程创徏独立的部|?span lang="EN-US">, 使得jar和类目录对于相应的统一cd载器(UnifiedClassLoader)可用. q个UnifiedClassLoader是注册到l一的类加蝲器仓库中?span lang="EN-US">.

3.      前缀1.3的方?span lang="EN-US">, 代表处理服务描述文g中定义的本地目录条目的过E?span lang="EN-US">. q个q程把在路径属性中指定?span lang="EN-US">SAR相关条目复制一份到server/<config>/db目录?span lang="EN-US">.

4.      Ҏ(gu)1.4, 代表对已l部|的服务中嵌套的可部单元的部|过E?span lang="EN-US">. 子部|项目被创徏q被加入到服务部|信息的子部|项列表?span lang="EN-US">.

5.      Ҏ(gu)2.1, SAR部v单元相应?span lang="EN-US">UnifiedClassLoader(本n也是MBean)被注册到MBean Server?span lang="EN-US">, q样它就可以被用来加?span lang="EN-US">SAR中的MBean?span lang="EN-US">.

6.      Ҏ(gu)2.2, 创徏XML服务描述文g中定义的每个MBeanq用描述文g中给定的值初始化其属?span lang="EN-US">. q些工作通过调用ServiceController?span lang="EN-US">install(Element, ObjectName)Ҏ(gu)来完成的.

7.      Ҏ(gu)2.4.1, 对于前面步骤中创建好的每?span lang="EN-US">MBean实例, 获取?span lang="EN-US">JMX对象名ƈ?span lang="EN-US">ServiceController处理服务生命周期中的create步骤. ServiceController处理MBean服务的依赖关p?span lang="EN-US">, 只有当所有依赖都满?span lang="EN-US">, createҎ(gu)才会(x)被调?span lang="EN-US">.

8.      前缀3.1的方?span lang="EN-US">, 代表MBean实例的v动过E?span lang="EN-US">. 对于创徏好的每个MBean实例, 获取?span lang="EN-US">JMX对象名ƈ?span lang="EN-US">ServiceController处理服务生命周期中的start步骤. ServiceController处理MBean服务的依赖关p?span lang="EN-US">, 只有当所有依赖都满?span lang="EN-US">, startҎ(gu)才会(x)被调?span lang="EN-US">.



sharajava 2006-07-25 11:18 发表评论
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走出ClassLoaderq局http://www.tkk7.com/sharajava/archive/2006/07/25/59946.htmlsharajavasharajavaTue, 25 Jul 2006 03:04:00 GMThttp://www.tkk7.com/sharajava/archive/2006/07/25/59946.htmlhttp://www.tkk7.com/sharajava/comments/59946.htmlhttp://www.tkk7.com/sharajava/archive/2006/07/25/59946.html#Feedback1http://www.tkk7.com/sharajava/comments/commentRss/59946.htmlhttp://www.tkk7.com/sharajava/services/trackbacks/59946.html ?/span> : 我什么时候应该?/span> Thread.getContextClassLoader()?

{?/span> : q个问题l常出现在编写框架代?/span> , 需要动态加载很多类和资源的时?/span> . 通常当你需要动态加载资源的时?/span> , 你至有三个 ClassLoader 可以选择 :

̔        pȝcd载器或叫作应用类加蝲?/span> (system classloader or application classloader)

̔        当前cd载器

̔        当前U程cd载器

上面的问题指的是最后一U类加蝲?/span> . 哪种cd载器是正的选择?/span> ?

W一U选择可以很容易地排除 : pȝcd载器 (system classloader). q个cd载器处理 -classpath 下的cd载工?/span> , 可以通过 ClassLoader.getSystemClassLoader() Ҏ(gu)调用 . ClassLoader 下所有的 getSystemXXX() 的静态方法都是通过q个Ҏ(gu)定义?/span> . 在你的代码中 , 你应该尽量少地调用这个方?/span> , 以其它的cd载器作ؓ(f)代理 . 否则你的代码只能工作在单的命o(h)行应用中 , q个时候系l类加蝲?/span> (system classloader) ?/span> JVM 最后创建的cd载器 . 一但你把代码移?/span> EJB, Web 应用?/span> Java Web Start 应用?/span> , 一定会(x)出问?/span> .

      所以我们来看第二种选择 : 当前上下文环境下的类加蝲?/span> . Ҏ(gu)定义 , 当前cd载器是你当前方法所属的cȝ加蝲?/span> . 在运行时cM间动态联~?/span> , ?qing)调?/span> Class.forName,() Class.getResource() {类似方法时 , q个cd载器?x)被隐含C?/span> . It is also used by syntactic constructs like X.class class literals.

    U程上下文类型加载器是在Java 2q_上被引入?span lang="EN-US">. 每一个线E都有一个类加蝲器与之对?span lang="EN-US">(除非q个U程是被本地代码创徏?span lang="EN-US">). q个cd载器是通过Thread.setContextClassLoaser()Ҏ(gu)讄?span lang="EN-US">. 如果你不在线E构造后调用q个Ҏ(gu), q个U程从它的父线E中l承相应的上下文cd载器. 如果在整个应用中你不做Q何特D设|?span lang="EN-US">, 所有的U程都以系l类加蝲?span lang="EN-US">(system classloader)作ؓ(f)自己的线E上下文cd载器. 自从Web?span lang="EN-US">J2EE应用服务器用成熟的cd载器机制来实现诸?span lang="EN-US">JNDI, U程?span lang="EN-US">, lg热部|等功能以来, q种在整个应用中不做MU程cd载器讄的情况就很少?span lang="EN-US">.

    Z么线E上下文cd载器存在于如此重要的位置?span lang="EN-US">? q个概念?span lang="EN-US">J2SE中的引入q不引h注目. 很多开发h员对q一概念qh的原因是Sun公司在这斚w~Z适当的指引和文.

    事实?span lang="EN-US">, 上下文类加蝲器提供了cd载机制的后门, q一点也?span lang="EN-US">J2SE中被引入?span lang="EN-US">. 通常, ?span lang="EN-US">JVM中的所有类加蝲器被l织成了有承层ơ的l构, 每一个类加蝲?span lang="EN-US">(除了引导JVM的原始类加蝲?span lang="EN-US">)都有一个父加蝲?span lang="EN-US">. 每当被请C加载类?span lang="EN-US">, cd载器都会(x)首先h其父cd载器, 只有当父cd载器不能加蝲?span lang="EN-US">, 才会(x)自己q行cd?span lang="EN-US">.

   有时候这U类加蝲的顺序安排不能正常工?span lang="EN-US">, 通常当必d态加载应用程序开发h员提供的资源的时?span lang="EN-US">. ?span lang="EN-US">JNDIZ: 它的内容(?span lang="EN-US">J2SE1.3开?span lang="EN-US">)在rt.jar中的引导cM实现?span lang="EN-US">, 但是q些JNDI核心c需要动态加载由独立厂商实现q|在应用E序?span lang="EN-US">classpath下的JNDI提供?span lang="EN-US">. q种情况p求一个父classloader(本例, 是引导cd载器)d载对于它其中一个子classloader(本例, pȝcd载器)可见的类. q时通常的类加蝲代理机制不能实现q个要求. 解决的办?span lang="EN-US">(workaround)是, ?span lang="EN-US">JNDI核心cM用当前线E上下文的类加蝲?span lang="EN-US">, q样, 基本的cd载代理机制的相反方向建立了一条有效的途径.

    另外, 上面一D可能让你想起一些其它的事情: XML解析Java API(JAXP). 是的, ?span lang="EN-US">JAXP只是J2SE的扩展进, 它很自然地用当前cd载器来引D析器的实?span lang="EN-US">. 而当JAXP被加入到J2SE1.4的核心类库中?span lang="EN-US">, 它的cd载也改成了用当前线E类加蝲?span lang="EN-US">, ?span lang="EN-US">JNDI的情况完全类?span lang="EN-US">(也很多E序员很qh). 明白Z么我说来?span lang="EN-US">Sun的指导很~Z了吧?

   在以上的介绍之后, 我们来看关键问题: q两U选择(当前cd载器和当前线E类加蝲?span lang="EN-US">)都不是在所有环境下都适用. 有些为当前线E类加蝲器应该成为新的标准策?span lang="EN-US">. 但是, 如果q样, 当多个线E通过׃n数据q行交互的时, 会(x)呈现Zq极其复杂的cd载的画面, 除非它们全部使用了同一个上下文的类加蝲?span lang="EN-US">. q一步说, 在某些遗留下来的解决Ҏ(gu)?span lang="EN-US">, 委派到当前类加蝲器的Ҏ(gu)已经是标?span lang="EN-US">. 比如?span lang="EN-US">Class.forName(String)的直接调?span lang="EN-US">(q也是我Z么推荐尽量避免对q个Ҏ(gu)q行调用的原?span lang="EN-US">). 即你努力去只调用上下文相关的类加蝲?span lang="EN-US">, 仍然?x)有一些代码会(x)不由你控?span lang="EN-US">. q种不受控制的类加蝲委派机制是؜入是很危险的.

    更严重的问题, 某些应用服务器把环境上下文及(qing)当前cd载器讄C同的cd载器实例?span lang="EN-US">, 而这些类加蝲器有相同的类路径但却没有委派机制中的父子关系. xqؓ(f)什么十分可?span lang="EN-US">. 要知道类加蝲器定义ƈ加蝲的类实例?x)带有一?span lang="EN-US">JVM内部?span lang="EN-US">ID?span lang="EN-US">. 如果当前cd载器加蝲一个类X的实?span lang="EN-US">, q个实例调用JNDI查找c?span lang="EN-US">Y的实?span lang="EN-US">, 些时的上下文的类加蝲器也可以定义了加载类Y实例. q个c?span lang="EN-US">Y的定义就与当前类加蝲器看到的c?span lang="EN-US">Y的定义不?span lang="EN-US">. 如果q行强制cd转换, 则生异?span lang="EN-US">.

   q种混ؕ的情况还在Java中存在一D|?span lang="EN-US">. 对于那些需要动态加载资源的J2SE?span lang="EN-US">API, 我们来猜惛_们的cd{略. 例如:

Ø         JNDI 使用U程上下文类加蝲?span lang="EN-US">

Ø         Class.getResource() ?span lang="EN-US">Class.forName()使用当前cd载器

Ø         JAXP(J2SE 1.4 ?qing)之?span lang="EN-US">)使用U程上下文类加蝲?span lang="EN-US">

Ø         java.util.ResourceBundle 使用调用者的当前cd载器

Ø         URL protocol handlers specified via java.protocol.handler.pkgs system property are looked up in the bootstrap and system classloaders only

Ø         Java 序列?span lang="EN-US">API默认使用调用者当前的cd载器

q些cd(qing)资源的加载策略问?span lang="EN-US">, 肯定?span lang="EN-US">J2SE领域中文最?qing)说明最~Z的部分了.



sharajava 2006-07-25 11:04 发表评论
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JBoss启动q程(?http://www.tkk7.com/sharajava/archive/2006/07/21/59332.htmlsharajavasharajavaFri, 21 Jul 2006 00:27:00 GMThttp://www.tkk7.com/sharajava/archive/2006/07/21/59332.htmlhttp://www.tkk7.com/sharajava/comments/59332.htmlhttp://www.tkk7.com/sharajava/archive/2006/07/21/59332.html#Feedback0http://www.tkk7.com/sharajava/comments/commentRss/59332.htmlhttp://www.tkk7.com/sharajava/services/trackbacks/59332.html 1.      org.jboss.Main.main(String[]) 为入?span lang="EN-US">.

2.      main 函数创徏一个名?span lang="EN-US">”jboss?/span>的线E组, 然后创徏一个属于该l的U程, 在线E中执行bootҎ(gu).

3.      boot Ҏ(gu)首先处理main函数中的参数(?qing)一些其它的pȝ环境讄), 接着qpȝ的属性创Zorg.jboss.system.server.ServerLoader实例[new ServerLoader(props)].

4.      ServerLoader 注册Jboss相关的类路径, 包括XML解析?span lang="EN-US">, jboss-jmx.jar, concurrent.jar?qing)其它的一些额外的c\?span lang="EN-US">.

5.      ServerLoader 通过load(ClassLoader)Ҏ(gu)创徏Jboss Server实例. 参数ClassLoader?span lang="EN-US">ClassLoader parentCL = Thread.currentThread(). getContextClassLoader( )得到的当前线E的cd载器. 创徏?span lang="EN-US">Server实例?span lang="EN-US" style="COLOR: #99cc00">org.jboss.system.server.Server接口的实?span lang="EN-US">. load(ClassLoader)Ҏ(gu)的细?span lang="EN-US">:

Ø        ?span lang="EN-US">jar包及(qing)?span lang="EN-US">ServerLoader中注册的c\径创Z?span lang="EN-US">URLClassLoader的实?span lang="EN-US">, 把传入的ClassLoader作ؓ(f)?span lang="EN-US">URLClassLoader?span lang="EN-US">parent.

Ø        Server 接口的实现类ql属?jboss.server.type军_, 默认?span lang="EN-US">      org.jboss.system.server.ServerImpl.

Ø        URLClassLoader 通过无参构造函数加?span lang="EN-US">Server接口实现的实?span lang="EN-US">. 在加载前把当前线E的cd载器|ؓ(f)?span lang="EN-US">URLClassLoader, 在加载完成后再置回之前传入的ClassLoader.

6.      Server 实例用系l属性进行初始化[server.init(props)].

7.      服务起动[server.start()]. 起动q程的默认实现如?span lang="EN-US">:

Ø        把当前线E类型加载器|ؓ(f)加蝲?span lang="EN-US">Server接口实现实例?span lang="EN-US">ClassLoader.

Ø        ?span lang="EN-US">jboss域内, 通过MBeanServerFactory?span lang="EN-US">createMBeanServer(String)Ҏ(gu)创徏MbeanServer实例.

Ø        ?span lang="EN-US">MBean Server上注?span lang="EN-US">ServerImpl?span lang="EN-US">ServerConfigImpl两个MBean.

Ø        初始化统一的类加蝲仓库(unified class loader repository), 用来装蝲服务器配|目录及(qing)其它可选目录下?span lang="EN-US">jar文g. 对于每一?span lang="EN-US">jar文g和类目录都会(x)创徏一个相应的org.jboss.jmx.loading.UnifiedClassLoader实例, q且注册到统一的仓库中. 其中一?span lang="EN-US">UnifiedClassLoader实例?x)被讄为当前线E上下文?span lang="EN-US">ClassLoader. [?: This effectively makes allUnifiedClassLoaders available through the thread context class loader.]

Ø        接下来创?span lang="EN-US" style="COLOR: #99cc00">org.jboss.system.ServiceController?span lang="EN-US">MBean实例. ServiceControllerJBoss MBean服务的生命周?span lang="EN-US">.

Ø        org.jboss.deployment.MainDeployer 实例被创建ƈ起动. MainDeployer理部v的依赖和部v的定?span lang="EN-US">.

Ø        org.jboss.deployment. JARDeployer 实例被创建ƈ起动. JARDeployer处理jar包的部v.

Ø        org.jboss.deployment. SARDeployer 实例被创建ƈ起动. SARDeployer处理JBoss MBean服务的部|?span lang="EN-US">.

Ø        MainDeployer 对当前服务器文g环境?span lang="EN-US">conf/jboss-service.xml定义的服务进行部|?span lang="EN-US">.

启动q程l束. 把当前线E上下文cd载器|回v动前?span lang="EN-US">ClassLoader.

sharajava 2006-07-21 08:27 发表评论
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