java Run-Time Data Areas JAVA运行时数据区

jvm运行时数据内存

以下内容摘录摘录自 Java Virtual Machine Specification 14版的 2.5章节

2.5 Run-Time Data Areas

The Java Virtual Machine defines various run-time data areas that are used during execution of a program. Some of these data areas are created on Java Virtual Machine start-up and are destroyed only when the Java Virtual Machine exits. Other data areas are per thread. Per-thread data areas are created when a thread is created and destroyed when the thread exits. 11

2.5.1 The pc Register

The Java Virtual Machine can support many threads of execution at once (JLS §17). Each Java Virtual Machine thread has its own pc (program counter) register. At any point, each Java Virtual Machine thread is executing the code of a single method,namely the current method (§2.6) for that thread.If that method is not native, the pc register contains the address of the Java Virtual Machine instruction currently being executed. If the method currently being executed by the thread is native, the value of the Java Virtual Machine’s pc register is undefined. The Java Virtual Machine’s pc register is wide enough to hold a returnAddress or a native pointer onthe specific platform.

2.5.2 Java Virtual Machine Stacks

Each Java Virtual Machine thread has a private Java Virtual Machine stack, created at the same time as the thread. A Java Virtual Machine stack stores frames (§2.6). A Java Virtual Machine stack is analogous to the stack of a conventional language such as C: it holds local variables and partial results, and plays a part in method invocation and return. Because the Java Virtual Machine stack is never manipulated directly except to push and pop frames, frames may be heap allocated. The memory for a Java Virtual Machine stack does not need to be contiguous.

In the First Edition of The Java® Virtual Machine Specification, the Java Virtual Machinestack was known as the Java stack.

This specification permits Java Virtual Machine stacks either to be of a fixed size or to dynamically expand and contract as required by the computation. If the Java Virtual Machine stacks are of a fixed size, the size of each Java Virtual Machine stack may be chosen independently when that stack is created.

A Java Virtual Machine implementation may provide the programmer or the user control over the initial size of Java Virtual Machine stacks, as well as, in the case of dynamically expanding or contracting Java Virtual Machine stacks, control over the maximum and minimum sizes.

The following exceptional conditions are associated with Java Virtual Machine stacks:

• If the computation in a thread requires a larger Java Virtual Machine stack than is permitted, the Java Virtual Machine throws a StackOverflowError.
• If Java VirtualMachine stacks can be dynamically expanded, and expansion is attempted but insufficient memory can be made available to effect the expansion,or if insufficient memory can be made available to create the initial Java Virtual Machine stack for a new thread, the Java Virtual Machine throws an OutOfMemoryError.

2.5.3 Heap

The Java Virtual Machine has a heap that is shared among all Java Virtual Machine threads.The heap is the run-time data area from which memory for all class instances and arrays is allocated. The heap is created on virtual machine start-up. Heap storage for objects is reclaimed by an automatic storage management system (known as a garbage collector); objects are never explicitly deallocated. The Java Virtual Machine assumes no particular type of automatic storage management system, and the storage management technique may be chosen according to the implementor’s system requirements. The heap may be of a fixed size or may be expanded as required by the computation and may be contracted if a larger heap becomes unnecessary. The memory for the heap does not need to be contiguous.

A Java Virtual Machine implementation may provide the programmer or the user control over the initial size of the heap, as well as, if the heap can be dynamically expanded or contracted,control over the maximum and minimum heap size.

The following exceptional condition is associated with the heap:

• If a computation requires more heap than can be made available by the automatic storage management system, the Java Virtual Machine throws an OutOfMemoryError.

2.5.4 Method Area

The Java Virtual Machine has a method area that is shared among all Java Virtual Machine threads. The method area is analogous to the storage area for compiled code of a conventional language or analogous to the “text” segment in an operating system process. It stores per-class structures such as the run-time constant pool, field and method data, and the code for methods and constructors, including the special methods used in class and interface initialization and in instance initialization (§2.9).
The method area is created on virtual machine start-up. Although the method area is logically part of the heap, simple implementations may choose not to either garbage collect or compact it. This specification does not mandate the location of the method area or the policies used to manage compiled code. The method area may be of a fixed size or may be expanded as required by the computation and may be contracted if a larger method area becomes unnecessary. The memory for the method area does not need to be contiguous.

A Java Virtual Machine implementation may provide the programmer or the user control over the initial size of the method area, as well as, in the case of a varying-size method area, control over the maximum and minimum method area size.

The following exceptional condition is associated with the method area:

• If memory in the method area cannot be made available to satisfy an allocation request, the Java Virtual Machine throws an OutOfMemoryError.

2.5.5 Run-Time Constant Pool

A run-time constant pool is a per-class or per-interface run-time representation of the constant_pool table in a class file (§4.4). It contains several kinds of constants, ranging from numeric literals known at compile-time to method and field references that must be resolved at run-time. The run-time constant pool serves a function similar to that of a symbol table for a conventional programming language, although it contains a wider range of data than a typical symbol table.
Each run-time constant pool is allocated from the Java Virtual Machine’s method area (§2.5.4). The run-time constant pool for a class or interface is constructed when the class or interface is created (§5.3) by the Java Virtual Machine.
The following exceptional condition is associated with the construction of the runtime constant pool for a class or interface:

• When creating a class or interface, if the construction of the run-time constant pool requires more memory than can be made available in the method area of the Java Virtual Machine, the Java Virtual Machine throws an OutOfMemoryError.

See §5 (Loading, Linking, and Initializing) for information about the construction of the run-time constant pool.

2.5.6 Native Method Stacks

An implementation of the Java Virtual Machine may use conventional stacks, colloquially called “C stacks,” to support native methods (methods written in a language other than the Java programming language). Native method stacks may also be used by the implementation of an interpreter for the Java Virtual Machine’s instruction set in a language such as C. Java Virtual Machine implementations that cannot load native methods and that do not themselves rely on conventional stacks need not supply native method stacks. If supplied, native method stacks are typically allocated per thread when each thread is created.
This specification permits native method stacks either to be of a fixed size or to dynamically expand and contract as required by the computation. If the native method stacks are of a fixed size, the size of each native method stack may be chosen independently when that stack is created.

A Java Virtual Machine implementation may provide the programmer or the user control over the initial size of the native method stacks, as well as, in the case of varying-size native method stacks, control over the maximum and minimum method stack sizes.

The following exceptional conditions are associated with native method stacks:

• If the computation in a thread requires a larger native method stack than is permitted, the Java Virtual Machine throws a StackOverflowError.
• If native method stacks can be dynamically expanded and native method stack expansion is attempted but insufficient memory can be made available, or if insufficient memory can be made available to create the initial native method stack for a new thread, the Java Virtual Machine throws an OutOfMemoryError.

总结:

根据jvms规范中的说明。
java在运行时的内存区域与对象分配。
区域分为了

1. PC program counter 程序计数器。 每个Java虚拟机线程都有自己的pc（程序计数器）寄存器。在任何时候，每个Java虚拟机线程都在执行单个方法的代码，即该线程的当前方法 。这个是线程独有的
2. stack 栈，每个Java线程都有一个私有栈，这个栈线程同时创建。这个栈里面存储的是栈帧 frames ，对于栈帧也是个重要的概念，再jvm规范中有专门规范栈帧的一章。
3. heap 堆，Jvm有一个堆，在所有线程之间共享 ，这个堆是运行时数据区域， 声明的所有的实例和数组都会存放在这里。
4. Method Area 方法区，JVM有一个方法区，是所有线程 共享的。他存储的是每个类的结构（Class对象），如运行时常量池、字段和方法数据，以及方法和构造函数的代码，包括在类和接口初始化以及实例初始化中使用的特殊方法

需要注意的是 method area 是逻辑概念。而对于method area再jvm中的实现 1.8版本前叫Perm Generation 永久空间 1.8之后叫Meta Space 元空间。
这两者的主要区别在于：
1. Perm Generation 存储字符串常量 ；MetaSpace 不存储字符串常量
2. Perm Generation FGC不会清理； MetaSpace 会被清理
3. Perm Generation 空间大小启动的时候指定，不能变 ；MetaSpace 不设定的话，最大就是物理内存

6. run-time constant pool 运行时常量池，这个常量池是从method area中获取的空间也算隶属于method area 空间。用于存放编译时就赋值好的数字 或者字符串 ，和运行时再被解析的方法和方法或者字段。
7. Native Method Stacks 本地方法栈，这里都是调用C或者C++方法的的JIN方法的空间。这个也是线程私有的。

针对此总结画图一份仅供参考：