Detailed discussion on Java memory area and memory overflow exception

1. Introduction

In the Java programming field, memory management seems to be automatically operated by virtual machines, and developers do not need to be over-involved. However, when memory leaks or overflows problems quietly emerge, if you lack in-depth understanding of the virtual machine memory operation mechanism, troubleshooting and repairing will be difficult. This article will conduct in-depth analysis of Java memory areas and discuss common memory overflow exceptions in detail.

2. Java runtime data area

(I) Program Counter

Function: The program counter is a small memory space, and its function is to indicate the line number of the current thread to execute bytecode. It is a key indicator of the program control flow and is responsible for branching, looping, jumping and other process controls.
characteristic: is private for threads. When a thread executes a Java method, the bytecode instruction address is recorded; when the local method is executed, the counter value is empty. This area will not appearOutOfMemoryError。

(II) Java virtual machine stack

Thread privateness: The Java virtual machine stack is also private threads, and its life cycle is closely connected to the threads.
Stack frame structure: Store local variable tables, operand stacks, dynamic connections and method exits when the method is executed in units of stack frames. The local variable table stores basic data types and object references, and determines its size during the compilation period.
Exception: May throw*Error(When the stack depth requested by the thread exceeds the allowable depth of the virtual machine) andOutOfMemoryError(There is no sufficient memory required when the stack is dynamically expanded).

(III) Local method stack

The relationship between functions and virtual machine stack: Functions are similar to virtual machine stack, mainly providing support for local method execution.
Implementation method: The implementation method is determined by the virtual machine.
Exception thrown: When stack depth overflows or expansion fails, it will be thrown*ErrorandOutOfMemoryError。

(IV) Java heap

Status and roles: It is the largest memory area managed by the virtual machine, shared by all threads, used to store object instances, and is the main area for garbage collection.
Generational collection theory: Regional division is based on the generational collection theory, and can be set to a fixed or extended size.
Memory overflow situation: Thrown when memory is insufficient and cannot be expanded OutOfMemoryError。

(V) Method area

Thread Sharing: Thread sharing area, used to store type information, constants, static variables, etc.
Historical changes: JDK 8 was often called "permanent generation" before, and then implemented using metaspace.
Exception: Thrown when memory is insufficientOutOfMemoryError。

(VI) Constant Pool during Runtime

Area: belongs to the method area, storing literals and symbolic references generated during the compilation period.
Dynamic: It is dynamic and can add constants during runtime.
Memory exception: Thrown when memory application failsOutOfMemoryError。

(VII) Direct memory

Special properties: Not a virtual machine specification defines the area.NIOIt can be used to allocate off-heap memory to improve performance.
Memory limit: Not limited by Java heap size, but is subject to native memory.
Overflow exception: Throw out when limit is exceeded OutOfMemoryError。

3. Extreme memory overflow practice

(I) Java heap overflow

Sample code: Exhaust heap memory by constantly creating objects.

import ;
import ;
class HeapOOM {
    static class OOMObject {}
    public static void main(String[] args) {
        List<OOMObject> list = new ArrayList<>();
        while (true) {
            (new OOMObject());
        }
    }
}

Solution: Can be passed-Xmx Parameters set the heap size and throw it when overflowingOutOfMemoryError: Java heap space, can be resized or optimized object creation logic to solve.

(II) Virtual machine stack overflow

Sample code: Use infinite recursion to make the stack depth exceed the limit.

class StackSOF {
    private static void stackLeak() {
        stackLeak();
    }
    public static void main(String[] args) {
        try {
            stackLeak();
        } catch (Throwable e) {
            ("Stack depth: " + ());
            ();
        }
    }
}

Solution: Throw out *Error, the recursive algorithm needs to be checked and the termination condition is set.

(III) Method area and runtime constant pool overflow

Sample code: Continuously add strings to the constant pool.

import ;
import ;
class MethodAreaOOM {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        int i = 0;
        while (true) {
            ((i++).intern());
        }
    }
}

Coping strategies: Throw out OutOfMemoryError, you need to pay attention to the usage of constant pools and avoid creating constants without restraint.

(IV) Direct memory overflow from this machine

Sample code: With the help ofUnsafeThe class continuously allocates direct memory.

import ;
import ;
class DirectMemoryOOM {
    private static final int _1MB = 1024 * 1024;
    public static void main(String[] args) throws IllegalAccessException {
        Field unsafeField = ()[0];
        (true);
        Unsafe unsafe = (Unsafe) (null);
        while (true) {
            (_1MB);
        }
    }
}

Solution: ThrowOutOfMemoryError, the virtual machine parameters need to be configured reasonably and the direct memory usage is monitored.

4. Conclusion

{
(_1MB);
}
}
}

**Solution**: Throw `OutOfMemoryError`, and the virtual machine parameters need to be configured reasonably and monitored for direct memory usage.

Deeply mastering the principles of Java memory area division and memory overflow exceptions is the key to the advancement of `Java` developers. In daily development, good memory management habits should be developed, and memory usage should be monitored with the help of tools to ensure stable and efficient operation of the program.

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