In-depth understanding of JAVA core garbage collection algorithm

In-depth understanding of JAVA core garbage collection algorithm requires specific code examples

Garbage Collection (GC) is a very important function in modern programming languages , it can automatically manage the allocation and release of memory, avoiding the complexity and possible errors of manual memory management. In the JAVA language, the garbage collection algorithm is one of the core functions of the JAVA Virtual Machine (JVM). Today we will have an in-depth understanding of the JAVA core garbage collection algorithm and look at some specific code examples.

1. The core principle of the garbage collection algorithm
In JAVA, the core principle of the garbage collection algorithm is to identify and release the memory space occupied by objects that are no longer used through marking and clearing.

1. Mark: The garbage collector first starts from the root object of the program, recursively traverses all reachable objects, and marks these objects as alive.
2. Sweep: The garbage collector scans the entire heap, recycles objects that are not marked as alive, and re-adds the recovered memory space to the available memory pool.

2. Specific implementation of garbage collection algorithm
The JAVA virtual machine uses different garbage collection algorithms to achieve automatic memory management. The following are commonly used garbage collection algorithms in JAVA virtual machines:

1. Mark and Sweep algorithm (Mark and Sweep): This is the most basic garbage collection algorithm. It first marks all live objects and then clears unmarked objects. The main problem with this algorithm is that it generates a lot of memory fragmentation.
   Code example:

public class Main {
    public static void main(String[] args) {
        // 创建对象
        Object obj1 = new Object();
        Object obj2 = new Object();
        
        // 设置obj1为obj2的引用
        obj2 = obj1;
        
        // 断开obj2对obj1的引用
        obj2 = null;
        
        // 执行垃圾回收
        System.gc();
    }
}

2. Copying algorithm (Copying): This is a garbage collection algorithm suitable for scenarios with low object survival rates. It divides the heap into two areas of the same size, and only uses one area at a time. When the memory usage in this area reaches a certain threshold, the surviving objects are copied to another area, and then the memory in the current area is cleared.
   Code example:

public class Main {
    public static void main(String[] args) {
        // 创建对象
        Object obj1 = new Object();
        Object obj2 = new Object();
        
        // 通过复制算法回收内存
        obj1 = null;
        
        // 执行垃圾回收
        System.gc();
    }
}

3. Mark-Compact algorithm (Mark and Compact): This is a kind of garbage that is suitable for both long-running and scenarios with many surviving objects. recycling algorithm. It first marks all live objects, then compresses the live objects to one end of the heap, and updates the reference addresses.
   Code example:

public class Main {
    public static void main(String[] args) {
        // 创建对象
        Object obj1 = new Object();
        Object obj2 = new Object();
        
        // 设置obj1为obj2的引用
        obj2 = obj1;
        
        // 断开obj2对obj1的引用
        obj2 = null;
        
        // 执行垃圾回收
        System.gc();
    }
}

3. Garbage collection trigger in the code example
In the above code example, use the System.gc() method to manually Trigger garbage collection. Although we can call this method explicitly, generally garbage collection is automatically triggered internally by the JVM.

4. Summary
Understanding and mastering the JAVA core garbage collection algorithm is very important for writing efficient JAVA programs. This article briefly introduces the core principles and specific implementation of garbage collection, and provides some sample code to help readers better understand the related concepts of garbage collection. I hope that through the explanation of this article, readers can have a deep understanding of JAVA's core garbage collection algorithm and be able to write high-quality JAVA code.

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