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This article focuses on "what is the returnAddress process in JVM". Interested friends may wish to have a look at it. The method introduced in this paper is simple, fast and practical. Now let the editor take you to learn "what is the returnAddress process in JVM"?

JVM is a virtual machine and a specification, which follows the design principle of von Neumann architecture. In the von Neumann architecture, it is pointed out that the data and instructions processed by the computer are binary numbers, stored in the same memory without distinction and executed sequentially, and the instructions are composed of opcodes and address codes. the opcode determines the operation type and the number type of the operation, and the address code indicates the address code and Operand. From dos to window8, from unix to ubuntu and CentOS, as well as MAC OS, different operating system instruction sets and data structures are different. JVM translates the same language to major operating systems by establishing a virtual machine on the operating system and defining a unified set of data structure and operation instructions. It can be said that JVM is the core of java. This is the essence that java can be compiled and run everywhere at once.

Data type

In JVM, data is divided into two main categories: primitive types (native type) and reference types (reference type).

Reference type, so that JVM can better support the design of object-oriented language, the value of reference type is used to point to the class instance or array allocated in memory. The JVM specification does not specify the implementation details of the reference type, such as how the reference should locate and access the objects in the heap, the specific object access method depends on the specific implementation of the virtual machine, for example, HotSpot has its own implementation scheme.

At present, the mainstream access methods are using handles and direct pointers:

Direct pointer access is used, which is similar to the virtual table in C++ (a virtual table is a pointer to object-type data). These two object access methods have their own advantages and disadvantages. The biggest advantage of using handle access is that the stable handle address is stored in reference. When the object is moved (such as garbage collection, sorting the memory space, it will move the storage location of the object), it will only change the pointer of the sample data in the handle, but the reference itself does not need to be modified.

The biggest advantage of using direct pointer access is that it is faster and saves the time cost of memory addressing.

Native data types include: numeric types, boolean type, returnAddress type. Among them, returnAddress data only exists at the bytecode level and has nothing to do with the programming language, that is to say, we will not directly deal with returnAddress type data in the Java language.

The value of returnAddress type is a pointer to bytecode. Whether it is a physical machine or a virtual machine, the data in memory at run time can be divided into two categories: code and data. For von Neumann computers, instruction data and numerical data are stored in memory, while Harvard computers store program instructions and data separately.

For JVM, a program is a bytecode instruction stored in the method area, and a value of type returnAddress is a pointer to a specific instruction memory address.

JVM supports multithreading, and each thread has its own program counter (pc register), and the value in pc is the memory address where the current instruction is located, that is, data of type returnAddress. When the thread executes the native method, the value in pc is undefined.

Stack frame

The stack frame (Stack Frame) is a data structure used to support the virtual machine to call and execute the method. The stack frame stores the local variable table of the method, Operand stack, dynamic connection, and method return address and other information. When the program is compiled, how many local variables are needed in the stack frame and how deep the Operand stack has been fully determined, and written in the Code attribute of the method table.

When a method starts to execute, there are only two ways to exit. The first is that the execution engine encounters a bytecode instruction returned by any method, which is called the normal completion exit; the other is that an exception is encountered during method execution and the exception is not caught, which is called the exception completion exit.

No matter which exit method, after the method exits, you need to return to the location (address) where the method was called and let the program continue to execute. In general, the value in the caller's current PC counter is saved before the method is executed, and when the method exits normally, the value of the PC counter is returned to the caller as the return address. When a method exits abnormally, the return address is determined by the exception handler table.

The process of exiting a method is actually equivalent to taking the current stack frame off the stack. The general process is as follows:

Restore the local variable table and Operand stack of the upper-level method

Press the return value into the Operand stack of the caller stack frame

Adjust the value of the PC counter to point to an instruction after the method call instruction

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