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What is the respective use of Java memory area? in view of this question, this article introduces the corresponding analysis and answer in detail, hoping to help more partners who want to solve this problem to find a more simple and feasible way.

Each memory area has its own purpose, as well as the creation and destruction time, some areas will exist with the startup of the virtual machine, and some areas will be established and destroyed depending on the user thread.

Program counter

The program counter (ProgramCounterRegister) is the private memory area of the thread, the line number indicator of the bytecode executed by the current thread, and a small piece of memory space.

When the bytecode interpreter (java source code is compiled into bytecode and run time is interpreted as machine code) works, it selects the next bytecode to be executed by changing the value of the program counter. Branches, loops, jumps, and so on all rely on program counters.

When a thread executes the Java method, the program counter records the address of the virtual machine bytecode instruction that is executing; when the thread executes the Native method, the value of the program counter is Undefined. The program counter is the only area of memory that does not specify OutOfMemoryError in the Java virtual machine specification.

Java virtual machine stack

The Java virtual machine stack (JavaStack) is also the private memory area of the thread, and its life cycle is the same as that of the thread. Virtual machine stack describes the memory model of Java method execution: each method creates a stack frame (StackFrame) to store local variables, Operand stack, dynamic links, method exit and other information. From the beginning of the call to the completion of the execution of each method, it corresponds to the process of a stack frame from the stack to the stack in the virtual machine stack.

The local variable table stores various basic data types (booleanbytecharshortintlongdoublefloat), object references, and returnAddress types (the address that points to a bytecode instruction) that are known at compile time. The memory space required by the local variable table is allocated during compilation. When entering a method, how much local variable space needs to be allocated in the stack frame is completely determined, and the size of the local variable table will not be changed during the operation of the method.

In the Java virtual machine specification, there are two possible exceptions to the Java virtual machine stack: StackOverflowError and OutOfMemoryError. If the depth of the stack requested by the thread is greater than the depth of the virtual machine stack, it will StackOverflowError. If not enough memory is requested when the virtual machine stack is dynamically expanded, it will be OutOfMemoryError.

Local method stack

The function of the local method stack (NativeMethodStack) is the same as that of the Java virtual machine stack, which is thread private. The difference is that the Java virtual machine stack serves to execute the Java method, and the local method stack serves the Native method. The virtual machine specification does not force the local method stack, so the local method stack and the virtual machine stack are combined into one in the HotSpot virtual machine. StackOverflowError and OutOfMemoryError are also thrown from this memory region.

Java reactor

The Java heap (JavaHeap) is an area of memory shared by all threads and the largest piece of memory managed by the Java virtual machine, which is created when the virtual machine starts. The sole purpose of this memory area is to store object instances, where almost all object instances allocate memory.

The Java heap is the main area of the garbage collector management heap, also known as the "GC heap". Now garbage collectors basically use generation-by-generation collection algorithms, so from the perspective of memory collection, Java heap can also be subdivided into: new generation and old age; new generation can be subdivided into Eden space, FromSurvivor space, ToSurvivor space.

The answer to the question about the respective use of the Java memory area is shared here. I hope the above content can be of some help to you. If you still have a lot of doubts to be solved, you can follow the industry information channel to learn more about it.

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