java虛擬機可能是下面三個中的一個
1:抽象規范
2:一個具體實現
3:一個虛擬機實例

java虛擬機的生命周期
java虛擬機的天職就是:運行一個java程序.當一個java程序運行開始運行時,一個虛擬機實例就產生了.當一個計算機上同時運行三個java程序.則將產生三個java虛擬機實例.每個程序運行在自己的虛擬機里面,不會干擾.當程序運行完畢時,虛擬機將自動退出.

java虛擬機里面有兩種線程,守護線程和非守護線程.守護線程是說java虛擬機自己的線程,如垃圾收集線程.而非守護線程則是java中運行的程序線程.當非守護線程都運行完了.java虛擬機將退出.

一個java虛擬機主要包括了:類轉載子系統,運行時數據區,執行引擎,內存區等等.

運行時數據區------主要是:1 堆 2? 方法區 3 java棧

堆和方法區對虛擬機實例中所有的對象都是共享的.而java棧區,是對每個線程都是獨立的. 當一個class被載入到 classloader中時,會解析它的類型信息.把這些類型信息放到方法區,而把程序中運行的對象,放到堆區.當一個新線程被創建,就分配一個新的java棧.java棧中保存的,是方法中的一些變量,狀態.java棧是由很多的java棧幀組成的.一個棧幀包含了一個方法運行的狀態.當一個方法被執行的時候,就壓入一個新的java棧幀到java棧中,方法返回的時候,就把棧幀彈出來,拋棄掉.


方法區

在java虛擬機中,被裝載的類的類型信息和類的靜態變量被存儲在方法區這樣的內存里面.java程序運行時,會查找這些個信息.方法區的大小,是動態的.也可以不是連續的.可自由在堆中分配.也可以由用戶或者程序員指定.方法區可被垃圾收集.

方法區可以保存以下信息
這個類型的全限定名
直接超類的全限定名
是類類型還是接口
類型的訪問修飾符
任何直接超類接口的全限定名的有序列表.
該類型的常量池
字段信息 類中聲明的每個字段及其順序 如字段名,類型.修飾符號.
方法信息:如方法名,返回類型.參數表列.修飾符號.字節碼.操作數棧和棧幀中局部變量區大小等等
類靜態變量
一個到類classloader的引用
一個到class類的引用


堆
用來存儲運行時的對象實例

java棧
每啟動一個新的線程.就會分配到一個java棧.java棧以幀為單位保存線程的運行狀態.它有兩種操作.入棧,出棧.
當一個方法被調用時,入棧,當一個方法返回時,出棧,或者當方法出現異常.也出棧.

棧幀
組成部分 局部變量區,操作數棧,幀數據區.
Eliminate obsolete object references消除過期的對象引用
When you switch from a language with manual memory management, such as C or C++, to agarbage-collected language, your job as a programmer is made much easier by the fact that your objects are automatically reclaimed when you're through with them. It seems almost like magic when you first experience it. It can easily lead to the impression that you don't have to think about memory management, but this isn't quite true.
java中也要注意內存管理

// Can you spot the "memory leak"?
public class Stack {
private Object[] elements;
private int size = 0;
public Stack(int initialCapacity) {
this.elements = new Object[initialCapacity];
}
Effective Java: Programming Language Guide
17
public void push(Object e) {
ensureCapacity();
elements[size++] = e;
}
public Object pop() {
if (size == 0)
throw new EmptyStackException();
return elements[--size];
}
/**
* Ensure space for at least one more element, roughly
* doubling the capacity each time the array needs to grow.
*/
private void ensureCapacity() {
if (elements.length == size) {
Object[] oldElements = elements;
elements = new Object[2 * elements.length + 1];
System.arraycopy(oldElements, 0, elements, 0, size);
}
}
}

這里有一個內存泄漏
如果一個棧先是增長，然后再收縮，那么，從棧中彈出來的對象將不會被當做垃圾回收，即使使用棧的客戶程序不再引用這些對象，它們也不會被回收。這是因為，棧內部維護著對這些對象的過期引用（obsolete re f e re n c e）。所謂過期引用，是指永遠也不會再被解除的引用

改正
public Object pop() {
if (size==0)
throw new EmptyStackException();
Object result = elements[--size];
elements[size] = null; // Eliminate obsolete reference清除引用
return result;
}

當程序員第一次被類似這樣的問題困擾的時候，他們往往會過分小心：對于每一個對象引用，一旦程序不再用到它，就把它清空。這樣做既沒必要，也不是我們所期望的，因為這樣做會把程序代碼弄得很亂，并且可以想像還會降低程序的性能。“清空對象引用”這樣的操作應該是一種例外，而不是一種規范行為。消除過期引用最好的方法是重用一個本來已經包含對象引用的變量，或者讓這個變量結束其生命周期。如果你是在最緊湊的作用域范圍內定義每一個變量（見第2 9條），則這種情形就會自然而然地發生。應該注意到，在目前的J V M實現平臺上，僅僅退出定義變量的代碼塊是不夠的，要想使引用消失，必須退出包含該變量的方法。

一般而言，只要一個類自己管理它的內存，程序員就應該警惕內存泄漏問題。一旦一個元素被釋放掉，則該元素中包含的任何對象引用應該要被清空。

內存泄漏的另一個常見來源是緩存。一旦你把一個對象引用放到一個緩存中，它就很容易被遺忘掉，從而使得它不再有用之后很長一段時間內仍然留在緩存中
Avoid creating duplicate objects避免創建重復的對象
It is often appropriate to reuse a single object instead of creating a new functionally equivalent object each time it is needed. Reuse can be both faster and more stylish. An object can always be reused if it is immutable

String s = new String("silly"); // DON'T DO THIS!

String s = "No longer silly";//do this

In addition to reusing immutable objects, you can also reuse mutable objects that you know will not be modified. Here is a slightly more subtle and much more common example of what not to do, involving mutable objects that are never modified once their values have been computed:
除了重用非可變的對象之外，對于那些已知不會被修改的可變對象，你也可以重用它們。

such as
public class Person {
private final Date birthDate;
// Other fields omitted
public Person(Date birthDate) {
this.birthDate = birthDate;
}
// DON'T DO THIS!
public boolean isBabyBoomer() {
Calendar gmtCal =
Calendar.getInstance(TimeZone.getTimeZone("GMT"));
gmtCal.set(1946, Calendar.JANUARY, 1, 0, 0, 0);
Date boomStart = gmtCal.getTime();
gmtCal.set(1965, Calendar.JANUARY, 1, 0, 0, 0);
Date boomEnd = gmtCal.getTime();
return birthDate.compareTo(boomStart) >= 0 &&
birthDate.compareTo(boomEnd) < 0;
}
}

改良的版本.
The isBabyBoomer method unnecessarily creates a new Calendar, TimeZone, and two Date instances each time it is invoked. The version that follows avoids this inefficiency with a static initializer:

class Person {
private final Date birthDate;
public Person(Date birthDate) {
this.birthDate = birthDate;
}
/**
* The starting and ending dates of the baby boom.
*/
private static final Date BOOM_START;
private static final Date BOOM_END;
static {
Calendar gmtCal =
Calendar.getInstance(TimeZone.getTimeZone("GMT"));
gmtCal.set(1946, Calendar.JANUARY, 1, 0, 0, 0);
BOOM_START = gmtCal.getTime();
gmtCal.set(1965, Calendar.JANUARY, 1, 0, 0, 0);
BOOM_END = gmtCal.getTime();
}
public boolean isBabyBoomer() {
return birthDate.compareTo(BOOM_START) >= 0 &&
birthDate.compareTo(BOOM_END) < 0;
}
}

。一個適配器是指這樣一個對象：它把功能委
托給后面的一個對象，從而為后面的對象提供一個可選的接口。由于適配器除了后面的對象之外，沒有其他的狀態信息，所以針對某個給定對象的特定適配器而言，它不需要創建多個適配器實例。

This item should not be misconstrued to imply that object creation is expensive and should be avoided. On the contrary, the creation and reclamation of small objects whose constructors do little explicit work is cheap, especially on modern JVM implementations. Creating additional objects to enhance the clarity, simplicity, or power of a program is generally a good thing.
Conversely, avoiding object creation by maintaining your own object pool is a bad idea unless the objects in the pool are extremely heavyweight. A prototypical example of an object that does justify an object pool is a database connection. The cost of establishing the connection is sufficiently high that it makes sense to reuse these objects. Generally speaking, however, maintaining your own object pools clutters up your code, increases memory footprint, and harms performance. Modern JVM implementations have highly optimized garbage collectors that easily outperform such object pools on lightweight objects.