How does GC in Java recycle objects?

Garbage Collection in Java: Object Recycling Mechanism

Introduction

Java uses automatic garbage The collection (GC) mechanism manages memory and releases objects that are no longer used. The GC tracks object references to determine which objects are no longer accessible and thus frees their memory.

Object reference

When an object is referenced by other objects, it survives. If no object refers to it, it is considered garbage. The GC collects and releases unreferenced garbage objects.

Key concepts of Java GC

• Reachability: Whether the object is referenced by other active objects.
• Root object: The initial object that is considered reachable, such as a global variable or a static variable.
• Garbage Collector: The thread that performs garbage collection operations.

How Java GC works

Java GC is generational, which divides the heap into different generations, and newly created objects enter the young generation. If an object survives in the young generation long enough, it will be promoted to the old generation. The GC will be more likely to collect objects in the young generation because there is more garbage there.

Practical case: Detecting garbage

You can use the System.gc() method to force GC. The following code example demonstrates how to detect garbage objects:

class MyClass {
    private static Object obj;

    public static void main(String[] args) {
        // 创建一个对象
        obj = new Object();

        // 将对该对象的引用设置为 null，使其成为垃圾
        obj = null;

        // 执行垃圾回收
        System.gc();

        // 尝试访问该对象（会抛出 NullPointerException）
        obj.hashCode();
    }
}

Conclusion

Java's garbage collection manages memory by tracking object references and releasing unused objects. It is a generational mechanism that performs garbage collection more frequently on newly created objects. By understanding this mechanism, you can optimize your code and avoid memory leaks.

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