How does the Java virtual machine use reference counting for memory management?

The Java virtual machine uses reference counting to manage memory usage. When the object's reference count reaches 0, the JVM will perform garbage collection. The reference counting mechanism includes: each object has a counter that stores the number of references pointing to the object. When an object is created, the reference counter is set to 1. When an object is referenced, the reference counter is incremented. When the reference ends, the reference counter is decremented.

Memory management of reference counting in Java virtual machine

Introduction

Java The virtual machine (JVM) uses reference counting to track the memory usage of objects. When an object's reference count reaches 0, the JVM will garbage collect it.

The principle of reference counting

Each Java object has a 32-bit reference counter, which stores the number of references pointing to the object. When an object is created, its reference counter is set to 1. When an object is referenced, its reference counter is incremented. When a reference ends, the reference counter is decremented.

Practical case

The following code example demonstrates how the Java virtual machine uses reference counting for memory management:

public class ReferenceCountingExample {
    public static void main(String[] args) {
        // 创建两个对象，并增加它们的引用计数
        Object object1 = new Object();
        Object object2 = new Object();
        object1 = null;  // 结束object1的引用
        // JVM 会垃圾回收object1，因为它的引用计数为0

        // 创建一个对object2的强引用
        Object strongReference = object2;
        // 创建一个对object2的弱引用
        WeakReference<Object> weakReference = new WeakReference<>(object2);

        // JVM 不会垃圾回收object2，因为还有强引用指向它
        object2 = null;  // 结束对object2的强引用
        // 执行垃圾回收
        System.gc();

        // JVM 会垃圾回收object2，因为现在只有弱引用指向它
        if (weakReference.get() == null) {
            System.out.println("object2 has been garbage collected");
        }
    }
}

In this code:

• object1 was garbage collected because it had no more references.
• object2 was not initially garbage collected because it had a strong reference pointing to it.
• When the strong reference is ended, the JVM performs garbage collection and object2 is released because it now only has a weak reference.

Advantages

• Reference counting is a simple and effective memory management technology.
• It can quickly detect unreferenced objects.

Disadvantages

• Reference counting may cause circular reference problems, leading to memory leaks.
• It requires maintaining a reference counter, which increases memory overhead.

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