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HttpAsyncClient

庄主渔R�� — Mon, 26 Nov 2012 02:46:00 GMT

HttpAsyncClient 是一个异步的 HTTP 客户端开发包�Q�基�?HttpCore NIO �?HttpClient �l��g�?/span>
HttpAsyncClient 的出现�ƈ不是��Z��替换 HttpClient�Q�而是作�ؓ(f��)一个补充用于需要大量�ƈ发连接，�Ҏ(gu��)��能要求非常高的��Z��HTTP的原生数据通信�Q�而且提供了事仉��动的 API�?br />
代码�C�Z��Q?br />

public Vector Gets(List urls)

throws IOReactorException, InterruptedException {

List requests = new ArrayList();

for (String url : urls) {

HttpGet get = new HttpGet(url);

requests.add(get);

}

final Vector dataPackages = new Vector();

HttpAsyncClient httpclient = new DefaultHttpAsyncClient();

httpclient.getParams().setIntParameter("http.socket.timeout", 5000)

.setIntParameter("http.connection.timeout", 5000)

.setIntParameter("http.socket.buffer-size", 8192)

.setBooleanParameter("http.tcp.nodelay", true);

final CountDownLatch latch = new CountDownLatch(requests.size());

httpclient.start();

try {

for (final HttpGet request : requests) {

httpclient.execute(request, new FutureCallback() {

@Override

public void completed(HttpResponse result) {

String statInfo = "";

try {

statInfo = result.getFirstHeader("statInfo").getValue();

if (statInfo != null) {

Long size = Long.parseLong(statInfo.split(",")[0].split(":")[1]);

dataPackages.add(size);

}

} catch (Exception e) {

System.out.println(e);

System.out.println(statInfo);

}

latch.countDown();

}

@Override

public void failed(Exception ex) {

latch.countDown();

}

@Override

public void cancelled() {

latch.countDown();

}

});

}

latch.await();

} finally {

httpclient.shutdown();

}

return dataPackages;

}

依赖的pom�Q?/span>httpasyncclient已经�?.0-beta3版本�?div>

org.apache.httpcomponents

httpasyncclient

4.0-beta1

org.apache.httpcomponents

httpclient

4.2-beta1

庄主渔R�� 2012-11-26 10:46 发表评论

web.xml��面配置

庄主渔R�� — Tue, 13 Nov 2012 03:37:00 GMT

摘要: web.xml��面配置参数详解加蝲��序与它们在 web.xml 文�g中的先后��序无关。即不会(x��)因�ؓ(f��) filter 写在 listener 的前面而会(x��)先加�?filter。最�l�得出的�l�论是：(x��)listener -> filter -> servlet�? 同时�q�存在着�q�样一�U�配�|�节... 阅读全文

庄主渔R�� 2012-11-13 11:37 发表评论

gc 对象晋升old区的一些规则�ȝ��

庄主渔R�� — Tue, 23 Oct 2012 12:52:00 GMT

ygc做步骤�ؓ(f��)�Q�清�I�eden+from中所有no ref的对象占用的内存

��eden+from中的所有存?g��u)zȝ��对象copy到to�?/div>

在这个过�E�中一些对象将晋升到old�?

--to放不下的

--存活�ơ数��过tenuring threshold�?/div>

重新计算Tenuring Threshold

然后谈谈MaxTenuringThreshold�q�个参数用于控制对象能经历多��次Minor GC才晋升到旧生代，默认值是15�Q�那是不是意味着对象要经�?5�ơminor gc才晋升到旧生代呢�Q�来看下面的一个例子�?/div>

public class GCTenuringThreshold{
public static void main(String[] args) throws Exception{
GCMemoryObject object1=new GCMemoryObject(2);
GCMemoryObject object2=new GCMemoryObject(8);
GCMemoryObject object3=new GCMemoryObject(8);
GCMemoryObject object4=new GCMemoryObject(8);
object2=null;
object3=null;
GCMemoryObject object5=new GCMemoryObject(8);
Thread.sleep(4000);
object2=new GCMemoryObject(8);
object3=new GCMemoryObject(8);
object2=null;
object3=null;
object5=null;
GCMemoryObject object6=new GCMemoryObject(8);
Thread.sleep(5000);
}
}
class GCMemoryObject{
private byte[] bytes=null;
public GCMemoryObject(int multi){
bytes=new byte[1024*256*multi];
}
}

�?Xms20M –Xmx20M –Xmn10M –XX:+UseSerialGC参数执行以上代码�Q�通过jstat -gcutil [pid] 1000 10的方式查看执行效�?/span>�Q�很惊讶执行�l�果竟然是在�W�二�ơminor GC的时候object1��p��晋升到old中了�Q�而可以肯定的是这个时�?span style="word-wrap: normal; word-break: normal; color: #ff0000; ">to space�I�间是充��的�Q�也��是说�ƈ不是在to space�I�间充��的情况下�Q�对象一定要�l�历MaxTenuringThreshold�ơ才�?x��)晋升到old�Q?/span>那具体规则到底是怎么��L(f��ng)��呢，��ȝ�� Hotspot 6 update 21中SerialGC的实玎ͼ�可以看到在每�ơminor GC后，�?x��)对�q�个存活周期的阈值做计算�Q�计��的代码如下�Q?br />
size_t desired_survivor_size = (size_t)((((double) survivor_capacity)*TargetSurvivorRatio)/100);
size_t total = 0;
int age = 1;
assert(sizes[0] == 0, "no objects with age zero should be recorded");
while (age total += sizes[age];
// check if including objects of age 'age' made us pass the desired
// size, if so 'age' is the new threshold
if (total > desired_survivor_size) break;
age++;
}
int result = age
其中desired_survivor_size是指survivor space/2�Q�从上面的代码可看出�Q�在计算存活周期�q�个阈值时�Q?span style="word-wrap: normal; word-break: normal; color: #ff0000; ">hotspot�?x��)遍历所有age的table�Q��ƈ对其所占用的大��进行篏�U�，当篏�U�的�?��超�q�了survivor space的一半时�Q�则以这个age作�ؓ(f��)新的存活周期阈��|��最后取age和MaxTenuringThreshold中更��的一个倹{�?/span>

按照�q�样的规则，上面的运行效果就可验证了�Q�第一�ơminor gc的时候存?g��u)z�d��期的阈��gؓ(f��)MaxTenuringThreshold�Q�minor gc�l�束后计��出新的阈��gؓ(f��)1�Q�在�W�二�ơminor gc时object 1的age已经�?了，因此object1被晋升到了旧生代�?/p>

�q�个规则对于Serial GC以及(qi��ng)ParNew GC�Q�但对于开启了UseAdaptiveSizePolicy的ParNew GC而言也无效，默认是不开启的�Q�均有效�Q�对于PS�Q�Parallel Scavenge�Q?GC而言�Q?span style="word-wrap: normal; word-break: normal; color: #ff0000; ">在默认的情况下第一�ơ以InitialTenuringThreshold�Q�默认�ؓ(f��)7�Q��ؓ(f��)准，之后在每�ơminor GC后均�?x��)动态计��，规则比上面的复杂�Q�在讄��-XX:-UseAdaptiveSizePolicy后，则以 MaxTenuringThrehsold为准�Q��ƈ且不�?x��)重新计��，会(x��)是恒定倹{�?/p>

如希望跟�t�每�ơminor GC后新的存?g��u)z�d��期的阈��|��可在启动参数上增加：(x��)-XX:+PrintTenuringDistribution�Q�输出的信息中的�Q?br />Desired survivor size 1048576 bytes, new threshold 7 (max 15)
new threshold 7��x��识新的存?g��u)z�d��期的阈��gؓ(f��)7�?/p>

庄主渔R�� 2012-10-23 20:52 发表评论

庄主渔R�� — Sun, 02 Sep 2012 16:48:00 GMT

原文链接�Q?a >http://java.sun.com/developer/technicalArticles/Programming/serialization/

We all know the Java platform allows us to create reusable objects in memory. However, all of those objects exist only as long as the Java virtual machine¹ remains running. It would be nice if the objects we create could exist beyond the lifetime of the virtual machine, wouldn't it? Well, with object serialization, you can flatten your objects and reuse them in powerful ways.

Object serialization is the process of saving an object's state to a sequence of bytes, as well as the process of rebuilding those bytes into a live object at some future time. The Java Serialization API provides a standard mechanism for developers to handle object serialization. The API is small and easy to use, provided the classes and methods are understood.

Throughout this article, we'll examine how to persist your Java objects, starting with the basics and proceeding to the more advanced concepts. We'll learn three different ways to perform serialization -- using the default protocol, customizing the default protocol, and creating our own protocol -- and we'll investigate concerns that arise with any persistence scheme such as object caching, version control, and performance issues.

By the conclusion of this article, you should have a solid comprehension of that powerful yet sometimes poorly understood Java API.

First Things First: The Default Mechanism

Let's start with the basics. To persist an object in Java, we must have a persistent object. An object is marked serializable by implementing thejava.io.Serializable interface, which signifies to the underlying API that the object can be flattened into bytes and subsequently inflated in the future.

Let's look at a persistent class we'll use to demonstrate the serialization mechanism:

10 import java.io.Serializable; 20 import java.util.Date; 30 import java.util.Calendar; 40 public class PersistentTime implements Serializable 50 { 60 private Date time; 70 80 public PersistentTime() 90 { 100 time = Calendar.getInstance().getTime(); 110 } 120 130 public Date getTime() 140 { 150 return time; 160 } 170 }

As you can see, the only thing we had to do differently from creating a normal class is implement the java.io.Serializable interface on line 40. The completely empty Serializable is only a marker interface -- it simply allows the serialization mechanism to verify that the class is able to be persisted. Thus, we turn to the first rule of serialization:

Rule #1: The object to be persisted must implement the Serializable interface or inherit that implementation from its object hierarchy.

The next step is to actually persist the object. That is done with the java.io.ObjectOutputStream class. That class is a filter stream--it is wrapped around a lower-level byte stream (called a node stream) to handle the serialization protocol for us. Node streams can be used to write to file systems or even across sockets. That means we could easily transfer a flattened object across a network wire and have it be rebuilt on the other side!

Take a look at the code used to save the PersistentTime object:

10 import java.io.ObjectOutputStream; 20 import java.io.FileOutputStream; 30 import java.io.IOException; 40 public class FlattenTime 50 { 60 public static void main(String [] args) 70 { 80 String filename = "time.ser"; 90 if(args.length > 0) 100 { 110 filename = args[0]; 120 } 130 PersistentTime time = new PersistentTime(); 140 FileOutputStream fos = null; 150 ObjectOutputStream out = null; 160 try 170 { 180 fos = new FileOutputStream(filename); 190 out = new ObjectOutputStream(fos); 200 out.writeObject(time); 210 out.close(); 220 } 230 catch(IOException ex) 240 { 250 ex.printStackTrace(); 260 } 270 } 280 }

The real work happens on line 200 when we call the ObjectOutputStream.writeObject() method, which kicks off the serialization mechanism and the object is flattened (in that case to a file).

To restore the file, we can employ the following code:

10 import java.io.ObjectInputStream; 20 import java.io.FileInputStream; 30 import java.io.IOException; 40 import java.util.Calendar; 50 public class InflateTime 60 { 70 public static void main(String [] args) 80 { 90 String filename = "time.ser"; 100 if(args.length > 0) 110 { 120 filename = args[0]; 130 } 140 PersistentTime time = null; 150 FileInputStream fis = null; 160 ObjectInputStream in = null; 170 try 180 { 190 fis = new FileInputStream(filename); 200 in = new ObjectInputStream(fis); 210 time = (PersistentTime)in.readObject(); 220 in.close(); 230 } 240 catch(IOException ex) 250 { 260 ex.printStackTrace(); 270 } 280 catch(ClassNotFoundException ex) 290 { 300 ex.printStackTrace(); 310 } 320 // print out restored time 330 System.out.println("Flattened time: " + time.getTime()); 340 System.out.println(); 350 // print out the current time 360 System.out.println("Current time: " + Calendar.getInstance().getTime()); 370 } 380}

In the code above, the object's restoration occurs on line 210 with the ObjectInputStream.readObject() method call. The method call reads in the raw bytes that we previously persisted and creates a live object that is an exact replica of the original. Because readObject() can read any serializable object, a cast to the correct type is required. With that in mind, the class file must be accessible from the system in which the restoration occurs. In other words, the object's class file and methods are not saved; only the object's state is saved.

Later, on line 360, we simply call the getTime() method to retrieve the time that the original object flattened. The flatten time is compared to the current time to demonstrate that the mechanism indeed worked as expected.

Nonserializable Objects

The basic mechanism of Java serialization is simple to use, but there are some more things to know. As mentioned before, only objects markedSerializable can be persisted. The java.lang.Object class does not implement that interface. Therefore, not all the objects in Java can be persisted automatically. The good news is that most of them -- like AWT and Swing GUI components, strings, and arrays -- are serializable.

On the other hand, certain system-level classes such as Thread, OutputStream and its subclasses, and Socket are not serializable. Indeed, it would not make any sense if they were. For example, thread running in my JVM would be using my system's memory. Persisting it and trying to run it in your JVM would make no sense at all. Another important point about java.lang.Object not implementing the Serializable interface is that any class you create that extends only Object (and no other serializable classes) is not serializable unless you implement the interface yourself (as done with the previous example).

That situation presents a problem: what if we have a class that contains an instance of Thread? In that case, can we ever persist objects of that type? The answer is yes, as long as we tell the serialization mechanism our intentions by marking our class's Thread object as transient.

Let's assume we want to create a class that performs an animation. I will not actually provide the animation code here, but here is the class we'll use:

10 import java.io.Serializable; 20 public class PersistentAnimation implements Serializable, Runnable 30 { 40 transient private Thread animator; 50 private int animationSpeed; 60 public PersistentAnimation(int animationSpeed) 70 { 80 this.animationSpeed = animationSpeed; 90 animator = new Thread(this); 100 animator.start(); 110 } 120 public void run() 130 { 140 while(true) 150 { 160 // do animation here 170 } 180 } 190 }

When we create an instance of the PersistentAnimation class, the thread animator will be created and started as we expect. We've marked the thread on line 40 transient to tell the serialization mechanism that the field should not be saved along with the rest of that object's state (in that case, the field speed). The bottom line: you must mark transient any field that either cannot be serialized or any field you do not want serialized. Serialization does not care about access modifiers such as private -- all nontransient fields are considered part of an object's persistent state and are eligible for persistence.

Therefore, we have another rule to add. Here are both rules concerning persistent objects:

Rule #1: The object to be persisted must implement the Serializable interface or inherit that implementation from its object hierarchy

Rule #2: The object to be persisted must mark all nonserializable fields transient

Customize the Default Protocol

Let's move on to the second way to perform serialization: customize the default protocol. Though the animation code above demonstrates how a thread could be included as part of an object while still making that object be serializable, there is a major problem with it if we recall how Java creates objects. To wit, when we create an object with the new keyword, the object's constructor is called only when a new instance of a class is created. Keeping that basic fact in mind, let's revisit our animation code. First, we instantiate an object of type PersistentAnimation, which begins the animation thread sequence. Next, we serialize the object with that code:

PersistentAnimation animation = new PersistentAnimation(10); FileOutputStream fos = ... ObjectOutputStream out = new ObjectOutputStream(fos); out.writeObject(animation);

All seems fine until we read the object back in with a call to the readObject() method. Remember, a constructor is called only when a new instance is created. We are not creating a new instance here, we are restoring a persisted object. The end result is the animation object will work only once, when it is first instantiated. Kind of makes it useless to persist it, huh?

Well, there is good news. We can make our object work the way we want it to; we can make the animation restart upon restoration of the object. To accomplish that, we could, for example, create a startAnimation() helper method that does what the constructor currently does. We could then call that method from the constructor, after which we read the object back in. Not bad, but it introduces more complexity. Now, anyone who wants to use that animation object will have to know that method has to be called following the normal deserialization process. That does not make for a seamless mechanism, something the Java Serialization API promises developers.

There is, however, a strange yet crafty solution. By using a built-in feature of the serialization mechanism, developers can enhance the normal process by providing two methods inside their class files. Those methods are:

private void writeObject(ObjectOutputStream out) throws IOException;

private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException;

Notice that both methods are (and must be) declared private, proving that neither method is inherited and overridden or overloaded. The trick here is that the virtual machine will automatically check to see if either method is declared during the corresponding method call. The virtual machine can call private methods of your class whenever it wants but no other objects can. Thus, the integrity of the class is maintained and the serialization protocol can continue to work as normal. The serialization protocol is always used the same way, by calling eitherObjectOutputStream.writeObject() or ObjectInputStream.readObject(). So, even though those specialized private methods are provided, the object serialization works the same way as far as any calling object is concerned.

Considering all that, let's look at a revised version of PersistentAnimation that includes those private methods to allow us to have control over the deserialization process, giving us a pseudo-constructor:

10 import java.io.Serializable; 20 public class PersistentAnimation implements Serializable, Runnable 30 { 40 transient private Thread animator; 50 private int animationSpeed; 60 public PersistentAnimation(int animationSpeed) 70 { 80 this.animationSpeed = animationSpeed; 90 startAnimation(); 100 } 110 public void run() 120 { 130 while(true) 140 { 150 // do animation here 160 } 170 } 180 private void writeObject(ObjectOutputStream out) throws IOException 190 { 200 out.defaultWriteObject(); 220 } 230 private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException 240 { 250 // our "pseudo-constructor" 260 in.defaultReadObject(); 270 // now we are a "live" object again, so let's run rebuild and start 280 startAnimation(); 290 300 } 310 private void startAnimation() 320 { 330 animator = new Thread(this); 340 animator.start(); 350 } 360 }

Notice the first line of each of the new private methods. Those calls do what they sound like -- they perform the default writing and reading of the flattened object, which is important because we are not replacing the normal process, we are only adding to it. Those methods work because the call to ObjectOutputStream.writeObject() kicks off the serialization protocol. First, the object is checked to ensure it implementsSerializable and then it is checked to see whether either of those private methods are provided. If they are provided, the stream class is passed as the parameter, giving the code control over its usage.

Those private methods can be used for any customization you need to make to the serialization process. Encryption could be added to the output and decryption to the input (note that the bytes are written and read in cleartext with no obfuscation at all). They could be used to add extra data to the stream, perhaps a company versioning code. The possibilities are truly limitless.

Stop That Serialization!

OK, we have seen quite a bit about the serialization process, now let's see some more. What if you create a class whose superclass is serializable but you do not want that new class to be serializable? You cannot unimplement an interface, so if your superclass does implementSerializable, your new class implements it, too (assuming both rules listed above are met). To stop the automatic serialization, you can once again use the private methods to just throw the NotSerializableException. Here is how that would be done:

10 private void writeObject(ObjectOutputStream out) throws IOException 20 { 30 throw new NotSerializableException("Not today!"); 40 } 50 private void readObject(ObjectInputStream in) throws IOException 60 { 70 throw new NotSerializableException("Not today!"); 80 }

Any attempt to write or read that object will now always result in the exception being thrown. Remember, since those methods are declaredprivate, nobody could modify your code without the source code available to them -- no overriding of those methods would be allowed by Java.

Create Your Own Protocol: the Externalizable Interface

Our discussion would be incomplete not to mention the third option for serialization: create your own protocol with the Externalizable interface. Instead of implementing the Serializable interface, you can implement Externalizable, which contains two methods:

public void writeExternal(ObjectOutput out) throws IOException;

public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException;

Just override those methods to provide your own protocol. Unlike the previous two serialization variations, nothing is provided for free here, though. That is, the protocol is entirely in your hands. Although it's the more difficult scenario, it's also the most controllable. An example situation for that alternate type of serialization: read and write PDF files with a Java application. If you know how to write and read PDF (the sequence of bytes required), you could provide the PDF-specific protocol in the writeExternal and readExternal methods.

Just as before, though, there is no difference in how a class that implements Externalizable is used. Just call writeObject() or readObjectand, voila, those externalizable methods will be called automatically.

Gotchas

There are a few things about the serialization protocol that can seem very strange to developers who are not aware. Of course, that is the purpose of the article -- to get you aware! So let's discuss a few of those gotchas and see if we can understand why they exist and how to handle them.

Caching Objects in the Stream

First, consider the situation in which an object is written to a stream and then written again later. By default, an ObjectOutputStream will maintain a reference to an object written to it. That means that if the state of the written object is written and then written again, the new state will not be saved! Here is a code snippet that shows that problem in action:

10 ObjectOutputStream out = new ObjectOutputStream(...); 20 MyObject obj = new MyObject(); // must be Serializable 30 obj.setState(100); 40 out.writeObject(obj); // saves object with state = 100 50 obj.setState(200); 60 out.writeObject(obj); // does not save new object state

There are two ways to control that situation. First, you could make sure to always close the stream after a write call, ensuring the new object is written out each time. Second, you could call the ObjectOutputStream.reset() method, which would tell the stream to release the cache of references it is holding so all new write calls will actually be written. Be careful, though -- the reset flushes the entire object cache, so all objects that have been written could be rewritten.

Version Control

With our second gotcha, imagine you create a class, instantiate it, and write it out to an object stream. That flattened object sits in the file system for some time. Meanwhile, you update the class file, perhaps adding a new field. What happens when you try to read in the flattened object?

Well, the bad news is that an exception will be thrown -- specifically, the java.io.InvalidClassException -- because all persistent-capable classes are automatically given a unique identifier. If the identifier of the class does not equal the identifier of the flattened object, the exception will be thrown. However, if you really think about it, why should it be thrown just because I added a field? Couldn't the field just be set to its default value and then written out next time?

Yes, but it takes a little code manipulation. The identifier that is part of all classes is maintained in a field called serialVersionUID. If you wish to control versioning, you simply have to provide the serialVersionUID field manually and ensure it is always the same, no matter what changes you make to the classfile. You can use a utility that comes with the JDK distribution called serialver to see what that code would be by default (it is just the hash code of the object by default).

Here is an example of using serialver with a class called Baz:

> serialver Baz > Baz: static final long serialVersionUID = 10275539472837495L;

Simply copy the returned line with the version ID and paste it into your code. (On a Windows box, you can run that utility with the - show option to simplify the copy and paste procedure.) Now, if you make any changes to the Baz class file, just ensure that same version ID is specified and all will be well.

The version control works great as long as the changes are compatible. Compatible changes include adding or removing a method or a field. Incompatible changes include changing an object's hierarchy or removing the implementation of the Serializable interface. A complete list of compatible and incompatible changes is given in the Java Serialization Specification.

Performance Considerations

Our third gotcha: the default mechanism, although simple to use, is not the best performer. I wrote out a Date object to a file 1,000 times, repeating that procedure 100 times. The average time to write out the Date object was 115 milliseconds. I then manually wrote out the Dateobject, using standard I/O the same number of iterations; the average time was 52 milliseconds. Almost half the time! There is often a trade-off between convenience and performance, and serialization proves no different. If speed is the primary consideration for your application, you may want to consider building a custom protocol.

Another consideration concerns the aforementioned fact that object references are cached in the output stream. Due to that, the system may not garbage collect the objects written to a stream if the stream is not closed. The best move, as always with I/O, is to close the streams as soon as possible, following the write operations.

Conclusion

Serialization in Java is simple to instigate and almost as simple to implement. Understanding the three different ways of implementing serialization should aid in bending the API to your will. We have seen a lot of the serialization mechanism in that article, and I hope it made things clearer and not worse. The bottom line, as with all coding, is to maintain common sense within the bounds of API familiarity. That article has laid out a strong basis of understanding the Java Serialization API, but I recommend perusing the specification to discover more fine-grained details.

庄主渔R�� 2012-09-03 00:48 发表评论

庄主渔R�� — Thu, 30 Aug 2012 13:29:00 GMT

摘要: 正则表达�? 引言正则表达式（regular expression�Q�就是用一�?#8220;字符�?#8221;来描�q�C��个特征，然后去验证另一�?#8220;字符�?#8221;是否�W�合�q�个特征。比如表辑ּ�“ab+” 描述的特征是“一�?nbsp;'a' �?nbsp;��L��?nbsp;'b... 阅读全文

庄主渔R�� 2012-08-30 21:29 发表评论