虚拟机基本构造

JVM主要包括两个子系统和两个组件。两个子系统分别是Class loader子系统和Execution engine(执行引擎) 子系统；两个组件分别是Runtime data area (运行时数据区域)组件和Native interface(本地接口)组件。

Class loader子系统的作用：根据给定的全限定名类名(如cc.ejb.example.HelloWorld)来装载class文件的内容到 Runtime data area中的method area(方法区域)。

Execution engine子系统的作用：执行classes中的指令。任何JVM specification实现(JDK)的核心都是Execution engine，不同的厂商有自己不同的实现。

Native interface组件：与native libraries交互，是其它编程语言交互的接口。当调用native方法的时候，就进入了一个全新的并且不再受虚拟机限制的世界，所以也很容易出现JVM无法控制的native heap OutOfMemory。

最后是Runtime Data Area组件：这就是我们常说的JVM的内存了。它主要分为五个部分——

1. Heap (堆)：一个Java虚拟实例中只存在一个堆空间
2. Method Area(方法区域)：被装载的class的信息存储在Method area的内存中。当虚拟机装载某个类型时，它使用类装载器定位相应的class文件，然后读入这个class文件内容并把它传输到虚拟机中。
3. Java Stack(java的栈)：虚拟机只会直接对Java stack执行两种操作：以帧为单位的压栈或出栈。
4. Program Counter(程序计数器)：每一个线程都有它自己的PC寄存器，也是该线程启动时创建的。PC寄存器的内容总是指向下一条将被执行指令的地址，这里的地址可以是一个本地指针，也可以是在方法区中相对应于该方法起始指令的偏移量。
5. Native method stack(本地方法栈)：保存native方法进入区域的地址

以上五部分只有Heap 和Method Area是被所有线程的共享使用的；而Java stack, Program counter 和Native method stack是以线程为粒度的，每个线程独自拥有自己的部分。

方法区

在JVM实例中，加载的类型的相关信息被保存在内存的一个逻辑区域中，称为方法区 method area。当JVM加载一个类型时，它用类装载器查找需要的Class文件。类装载器读入Class文件并以二进制数据的线性流传递给VM。 虚拟机从二进制数据中提取有关类型的信息，并将之保存在方法区中。而保存类里定义的类变量（静态变量）的内存被从方法区中拿走。下面的信息存储在方法区中：

• 类型信息
  The fully qualified name of the type
  The fully qualified name of the type’s direct superclass (unless the type is an interface or class java.lang.Object, neither of which have a superclass)
  Whether or not the type is a class or an interface
  The type’s modifiers ( some subset of` public, abstract, final)
  An ordered list of the fully qualified names of any direct superinterfaces

The Constant Pool
For each type it loads, a Java virtual machine must store a constant pool. A constant pool is an ordered set of constants used by the type, including literals (string, integer, and floating point constants) and symbolic references to types, fields, and methods.

Field Information
For each field declared in the type, the following information must be stored in the method area:

The field’s name
The field’s type
The field’s modifiers (some subset of public, private, protected, static, final, volatile, transient)

Method Information
For each method declared in the type, the following information must be stored in the method area:

The method’s name
The method’s return type (or void)
The number and types (in order) of the method’s parameters
The method’s modifiers (some subset of public, private, protected, static, final, synchronized, native, abstract)
An exception table

Class Variables
Class variables are shared among all instances of a class and can be accessed even in the absence of any instance. These variables are associated with the class–not with instances of the class–so they are logically part of the class data in the method area. Before a Java virtual machine uses a class, it must allocate memory from the method area for each non-final class variable declared in the class.

Constants (class variables declared final) are not treated in the same way as non-final class variables. Every type that uses a final class variable gets a copy of the constant value in its own constant pool. As part of the constant pool, final class variables are stored in the method area–just like non-final class variables. But whereas non-final class variables are stored as part of the data for the type that declares them, final class variables are stored as part of the data for any type that uses them.

A Reference to Class ClassLoader
For each type it loads, a Java virtual machine must keep track of whether or not the type was loaded via the bootstrap class loader or a user-defined class loader. For those types loaded via a user-defined class loader, the virtual machine must store a reference to the user-defined class loader that loaded the type. This information is stored as part of the type’s data in the method area.

A Reference to Class Class
An instance of class java.lang.Class is created by the Java virtual machine for every type it loads. The virtual machine must in some way associate a reference to the Class instance for a type with the type’s data in the method area.

Method Tables
For each non-abstract class a Java virtual machine loads, it could generate a method table and include it as part of the class information it stores in the method area. A method table is an array of direct references to all the instance methods that may be invoked on a class instance, including instance methods inherited from superclasses. A method table allows a virtual machine to quickly locate an instance method invoked on an object.

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