JVM学习笔记-帧数据区(Frame Data)

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概述

In addition to the local variables and operand stack, the Java stack frame includes data to support constant pool resolution, normal method return, and exception dispatch. This data is stored in the frame data portion of the Java stack frame.

除了局部变量区和操作数栈外，Java栈帧还需要一些数据来支持常量池解析、正常方法返回以及异常派发机制。这些信息都保存在Java栈帧的帧数据区中。

Many instructions in the Java Virtual Machine’s instruction set refer to entries in the constant pool. Some instructions merely push constant values of type int, long, float, double, or String from the constant pool onto the operand stack. Some instructions use constant pool entries to refer to classes or arrays to instantiate, fields to access, or methods to invoke. Other instructions determine whether a particular object is a descendant of a particular class or interface specified by a constant pool entry.

Java虚拟机中的大多数指令都涉及到常量池入口。有些指令仅仅是从常量池中取出数据然后压入Java栈（这些数据的类型包括int,long、float、double和String）；还有些指令使用常量池的数据来指示要实例化的类或数组，要访问的字段，或要调用的方法；还有些指令需要常量池中的数据才能确定某个对象是否属于某个类或实现了某个接口。

Whenever the Java Virtual Machine encounters any of the instructions that refer to an entry in the constant pool, it uses the frame dataís pointer to the constant pool to access that information. As mentioned earlier, references to types, fields, and methods in the constant pool are initially symbolic. When the virtual machine looks up a constant pool entry that refers to a class, interface, field, or method, that reference may still be symbolic. If so, the virtual machine must resolve the reference at that time.

每当虚拟机要执行某个需要用到常量池数据的指令时，它都会通过帧数据区中指向常量池的指针来访问它。以前讲过，常量池中对类型、字段和方法的引用在开始时都是符号。当虚拟机在常量池中搜索的时候，如果遇到指向类、接口、字段或者方法的入口，假若它们仍然是符号，虚拟机那时才会（也必须）进行解析。

Aside from constant pool resolution, the frame data must assist the virtual machine in processing a normal or abrupt method completion. If a method completes normally (by returning), the virtual machine must restore the stack frame of the invoking method. It must set the pc register to point to the instruction in the invoking method that follows the instruction that invoked the completing method. If the completing method returns a value, the virtual machine must push that value onto the operand stack of the invoking method.

除了用于常量池的解析外，帧数据区还要帮助虚拟机处理Java方法的正常结束或异常中止。如果是通过return正常结束，虚拟机必须恢复发起调用的方法的栈帧，包括设置PC寄存器指向发起调用的方法中的指令——即紧跟着调用了完成方法的指令的下一个指令。假如方法有返回值，虚拟机必须将它压入到发起调用的方法的操作数栈。

The frame data must also contain some kind of reference to the method’s exception table, which the virtual machine uses to process any exceptions thrown during the course of execution of the method. An exception table, which is described in detail in Chapter 17, "Exceptions," defines ranges within the bytecodes of a method that are protected by catch clauses. Each entry in an exception table gives a starting and ending position of the range protected by a catch clause, an index into the constant pool that gives the exception class being caught, and a starting position of the catch clauseís code.

为了处理Java方法执行期间的异常退出情况，帧数据区还必须保存一个对此方法异常表的引用。异常表定义了在这个方法的字节码中受catch子句保护的范围，异常表中的每一项都有一个被catch子句保护的代码的起始和结束位置（译者注：即try子句内部的代码），可能被catch的异常类在常量池中的索引值，以及catch子句内的代码开始的位置。

When a method throws an exception, the Java Virtual Machine uses the exception table referred to by the frame data to determine how to handle the exception. If the virtual machine finds a matching catch clause in the method’s exception table, it transfers control to the beginning of that catch clause. If the virtual machine doesnít find a matching catch clause, the method completes abnormally. The virtual machine uses the information in the frame data to restore the invoking methodís frame. It then rethrows the same exception in the context of the invoking method.

当某个方法抛出异常时，虚拟机根据帧数据区对应的异常表来决定如何处理。如果在异常表中找到了匹配的catch子句，就会把控制权转交给catch子句内的代码。如果没有发现，方法会立即异常中止。然后虚拟机使用帧数据区的信息回复发起调用的方法的帧，然后在发起调用的方法的上下文中重新抛出同样的异常。

In addition to data to support constant pool resolution, normal method return, and exception dispatch, the stack frame may also include other information that is implementation dependent, such as data to support debugging.

除了上述信息（支持常量池解析、正常方法返回和异常派发的数据）外，虚拟机的实现者也可以将其他信息放入帧数据区，如用于调试的数据等。