The Structure of the Java Virtual Machine(java虚拟机结构之运行时数据区翻译)

青旅半醒 2023-02-17 15:52 4阅读 0赞

### he Structure of the Java Virtual Machine ###

*   *  Run-Time Data Areas (运行时数据区)
     *   *  The pc Register （程序计数器）
         *  Java Virtual Machine Stacks （java虚拟机栈）
         *  Heap（堆）
         *  Method Area（方法区）
         *  Run-Time Constant Pool (运行时常量池)
         *  Native Method Stacks（本地方法栈）

## Run-Time Data Areas (运行时数据区) ##

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.  
译文：Java虚拟机定义了在程序执行期间使用的各种运行时数据区域。  
其中一些数据区域是在Java虚拟机启动时创建的，只在Java虚拟机退出时销毁，其他数据区域为每个线程。每个线程数据区域在线程创建时创建，在线程退出时销毁。

### The pc Register （程序计数器） ###

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 on the specific platform.  
译文:Java虚拟机可以支持同时执行多个线程(JLS§17)。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(§2.6)。如果该方法不是本机的，则pc寄存器包含当前正在执行的Java虚拟机指令的地址。如果线程当前执行的方法是本地的，那么Java虚拟机的pc寄存器的值是未定义的。Java虚拟机的pc寄存器足够宽，可以在特定平台上保存一个returnAddress或一个本机指针。

### Java Virtual Machine Stacks （java虚拟机栈） ###

### Heap（堆） ###

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.  
译文:Java虚拟机有一个堆，在所有Java虚拟机线程之间共享。堆是运行时数据区域，为所有类实例和数组分配内存。堆在虚拟机启动时创建。对象的堆存储由自动存储管理系统(称为垃圾收集器)回收;对象永远不会显式地释放，Java虚拟机假设没有特定类型的自动存储管理系统，存储管理技术可以根据实施者的系统需求进行选择。堆的大小可以是固定的，也可以根据计算的需要进行扩展，如果不必要的话，还可以收缩，堆的内存不需要是连续的。Java虚拟机实现可以让程序员或用户控制堆的初始大小，如果堆可以动态扩展或收缩，还可以控制堆的最大和最小大小。

下面是与堆相关的异常情况:  
如果计算需要的堆比自动存储管理系统提供的堆多，Java虚拟机会抛出OutOfMemoryError。

### 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 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 (§2.9) used in class and instance initialization and interface 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 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.  
译文：  
Java虚拟机有一个在所有Java虚拟机线程之间共享的方法区域。方法区域类似于传统语言编译代码的存储区域，或类似于操作系统进程中的“文本”段。它存储每个类的结构，比如运行时常量池、字段和方法数据，以及方法和构造函数的代码，包括在类和实例初始化以及接口初始化中使用的特殊方法(§2.9)。  
方法区域是在虚拟机启动时创建的。虽然方法区域在逻辑上是堆的一部分，但简单实现可能选择不进行垃圾收集或压缩。此规范不强制规定用于管理已编译代码的方法区域或策略的位置。方法区域可以是固定大小，也可以根据计算的需要进行扩展，如果不需要更大的方法区域，则可以收缩。方法区域的内存不需要是连续的。  
Java虚拟机实现可以为程序员或用户提供对方法区域初始大小的控制，以及在方法区域大小可变的情况下，对最大和最小方法区域大小的控制。  
以下异常情况与方法区域相关:  
如果方法区域中的内存无法满足分配请求，Java虚拟机将抛出OutOfMemoryError错误。

### Run-Time Constant Pool (运行时常量池) ###

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 run-time 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.  
译文：  
运行时常量池是类文件中constant\_pool表的每个类或每个接口的运行时表示(§4.4)。它包含几种类型的常量，从编译时已知的数值文字到必须在运行时解析的方法和字段引用。运行时常量池提供的功能与传统编程语言的符号表类似，尽管它包含的数据范围比典型的符号表更广。  
每个运行时常量池都是从Java虚拟机的方法区分配的(§2.5.4)。类或接口的运行时常量池是在Java虚拟机创建类或接口时构建的(§5.3)。  
以下异常情况与类或接口的运行时常量池的构建相关:  
在创建类或接口时，如果构建运行时常量池需要的内存超过了Java虚拟机的方法区域可用的内存，那么Java虚拟机将抛出OutOfMemoryError。  
参见§5(加载、链接和初始化)获取关于构建运行时常量池的信息。

### 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.

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.  
译文：  
Java虚拟机的实现可以使用传统的栈(通俗地称为“C栈”)来支持本地方法(用Java编程语言以外的语言编写的方法)。本地方法栈的实现也可以使用一个翻译为Java虚拟机的指令集的语言如c . Java虚拟机实现,无法加载本地方法,自己不依赖传统的本地方法栈栈不需要供应。如果提供了本机方法栈，则通常在创建每个线程时为每个线程分配本机方法栈。  
该规范允许本地方法堆栈具有固定的大小，或者根据计算的需要动态地展开和收缩。如果本机方法堆栈的大小是固定的，那么可以在创建该堆栈时独立选择每个本机方法堆栈的大小。  
Java虚拟机实现可以为程序员或用户提供对原生方法栈初始大小的控制，并且，在原生方法栈大小可变的情况下，还可以控制最大和最小方法栈大小。  
下面的异常条件与本机方法栈相关联

如果线程中的计算需要比允许的更大的本机方法堆栈，Java虚拟机将抛出StackOverflowError。  
如果可以动态扩展本机方法堆栈，并且尝试扩展本机方法堆栈，但是可用内存不足，或者如果可用内存不足，可以为新线程创建初始本机方法堆栈，Java虚拟机将抛出OutOfMemoryError