java 虚拟机运行时数据区域 Runtime Data Areas
The Java virtual machine defines various runtime 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.
虚拟机定义了各种运行时数据区以备程序运行时利用。一部份在虚拟机启动时被创建，在虚拟机退出时被销毁。另一部份跟线程有关，在线程创建时被创建，线程退出时被销毁。
3.5.1 The pc Register 寄存器
The Java virtual machine can support many threads of execution at once (§2.19). 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, the current method (§3.6) 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 on the specific platform.
虚拟机支持多线程，每一个线程都拥有自己的寄存器。在任一点，每个虚拟机线程都执行特定代码，当前方法。如果此方法不适本地的，
3.5.2 Java Virtual Machine Stacks 虚拟机栈
Each Java virtual machine thread has a private Java virtual machine stack, created at the same time as the thread.3 A Java virtual machine stack stores frames (§3.6). 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.
The Java virtual machine 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.4
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 virtual machine 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.
3.5.3 Heap 堆
The Java virtual machine has a heap that is shared among all Java virtual machine threads. The heap is the runtime data area from which memory for all class instances and arrays is allocated.
虚拟机有一个被所有虚拟机线程共享的堆。所有类实例和数组分配的内存都是在堆中分配的。
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.5
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.
3.5.4 Method Area 方法区
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 a UNIX process. It stores per-class structures such as the runtime constant pool, field and method data, and the code for methods and constructors, including the special methods (§3.9) used in class and instance initialization and interface type initialization.
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 version of the Java virtual machine 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.6
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.
3.5.5 Runtime Constant Pool 运行时常量池
A runtime constant pool is a per-class or per-interface runtime 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 runtime 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 runtime constant pool is allocated from the Java virtual machine's method area (§3.5.4). The runtime 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 runtime 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 Chapter 5 for information about the construction of the runtime constant pool.
3.5.6 Native Method Stacks 本地方法栈
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.
The Java virtual machine 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. In any case, a Java virtual machine implementation may provide the programmer or the user control over the initial size of the native method stacks. In the case of varying-size native method stacks, it may also make available control over the maximum and minimum method stack sizes.7
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.