Java interview memory model

SF interview question: Do you understand the Java memory model?

360 interview question: Distribution of program memory, five parts

(Learning video sharing: java teaching video)

1. The execution process of the java program

The Java source code file (.java suffix) will be compiled into a bytecode file (.class suffix) by the Java compiler, and then used by the JVM The class loader loads the bytecode files of each class. After the loading is completed, it is handed over to the JVM execution engine for execution. During the entire program execution process, the JVM will use a space to store data and related information needed during program execution. This space is generally called the Runtime Data Area, which is what we often call the JVM. Memory. Therefore, the memory management we often talk about in Java is to manage this space (how to allocate and reclaim memory space).

(Recommendations for more related interview questions: java interview questions and answers)

2. What parts does the runtime data area include:

Method Area:

The method area is a memory area shared by each thread. It is used to store class information, constants, and static information that have been loaded by the virtual machine. Variables, code compiled by just-in-time compiler and other data. When the method area cannot meet the memory allocation requirements, an OutOfMemoryError exception will be thrown.
The method area stores the class version, fields, methods, interfaces and constant pools. The constant pool stores literal and symbolic references. Symbol references include: 1. Fully qualified name of the class, 2. Field names and attributes, 3. Method names and attributes.

JVM Heap (Java Heap):

The Java heap is also a memory area shared by threads. It is created when the virtual machine starts and is managed by the Java virtual machine. The largest piece of memory, mainly used to store object instances. Almost all object instances allocate memory here. Note that the Java heap is the main area managed by the garbage collector, so it is often called the GC heap. If it is in the heap When no memory is available for instance allocation and the heap cannot be expanded, an OutOfMemoryError exception is thrown.

Program Counter Register:

When the bytecode interpreter is working, it selects the next bytecode that needs to be executed by changing the value of this counter. Instructions, branches, loops, jumps, exception handling, thread recovery and other basic functions all need to rely on this counter to complete.
In multi-threading, in order to restore the correct execution position after thread switching, each thread needs to have an independent program counter. Each thread does not affect each other and is stored independently. Therefore, this memory is a thread private.

Virtual machine stack (Java Virtual Machine Stacks):

The Java virtual machine stack is also private to the thread, and its life cycle is the same as the thread. The virtual machine stack describes the memory model of Java method execution: when each method is executed, a stack frame is created to store local variable tables, operand stacks, dynamic linked lists, method exit information, etc. The process from the call to the completion of execution of each method corresponds to the process from pushing a stack frame into the virtual machine stack to popping it out.

Native Method Stacks:

The local method stack belongs to the thread private data area. This part is mainly related to the Native methods used by the virtual machine. Normally, we don't need to care about this area.

(Related recommendations: java introductory tutorial)

The above is the detailed content of Java interview memory model. For more information, please follow other related articles on the PHP Chinese website!