Analysis of JVM garbage collection algorithm: explore its characteristics

JVM Garbage Collection Algorithm Analysis: To explore what it is, specific code examples are needed

Abstract:
JVM (Java Virtual Machine) is the execution of Java applications environment, and the garbage collection mechanism is one of the important components of the JVM. This article will analyze the JVM garbage collection algorithm, introduce its common algorithm types, and illustrate the application of various algorithms through specific code examples.

1. Introduction
   Garbage Collection is a technology that automatically manages memory. It can identify objects that are no longer used and release the memory they occupy. In Java, garbage collection is performed by the JVM. The goal of the JVM garbage collection algorithm is to minimize the pause time and memory usage of the application and improve the performance and availability of the system.
2. Common Garbage Collection Algorithms
   2.1 Mark-Sweep Algorithm (Mark and Sweep)
   The Mark-Sweep algorithm is one of the most basic garbage collection algorithms. It works by first marking all living objects through the root node (such as global variables, static variables, etc.), and then scanning and clearing unmarked objects from the heap. The disadvantage of this algorithm is that it generates a lot of memory fragmentation.

Sample code:

public class MarkAndSweepAlgorithm {
    private boolean isMarked;
    
    public static void main(String[] args) {
        MarkAndSweepAlgorithm obj1 = new MarkAndSweepAlgorithm();
        MarkAndSweepAlgorithm obj2 = new MarkAndSweepAlgorithm();
        // obj1和obj2被引用，是存活对象
        obj1.isMarked = true;
        obj2.isMarked = true;
        // ...
        // 执行垃圾回收
        // ...
        // 标记所有存活的对象
        // ...
        // 清除未被标记的对象
        // ...
    }
}

2.2 Copying algorithm (Copying)
The copying algorithm divides the available memory into two equal-sized areas, using only half of them each time. When half of the memory is used up, the surviving objects are copied to the other half of the memory, and then all objects in the original memory are cleared. The advantage of this algorithm is that it is simple and efficient, and is suitable for scenarios with high memory usage.

Sample code:

public class CopyingAlgorithm {
    public static void main(String[] args) {
        CopyingAlgorithm obj1 = new CopyingAlgorithm();
        CopyingAlgorithm obj2 = new CopyingAlgorithm();
        // obj1和obj2被引用，是存活对象
        // ...
        // 执行垃圾回收
        CopyingAlgorithm obj3 = obj1;
        obj1 = obj2;
        obj2 = obj3;
        // obj1和obj2存活，obj3被回收
    }
}

2.3 Mark-compression algorithm (Mark and Compact)
The mark-compression algorithm is improved on the basis of the mark-clear algorithm. It works by first marking all living objects, then compressing these objects to one end of the memory, and cleaning up unmarked objects. This avoids memory fragmentation problems.

Sample code:

public class MarkAndCompactAlgorithm {
    private boolean isMarked;
    
    public static void main(String[] args) {
        MarkAndCompactAlgorithm obj1 = new MarkAndCompactAlgorithm();
        MarkAndCompactAlgorithm obj2 = new MarkAndCompactAlgorithm();
        // obj1和obj2被引用，是存活对象
        obj1.isMarked = true;
        obj2.isMarked = true;
        // ...
        // 执行垃圾回收
        // ...
        // 标记所有存活的对象
        // ...
        // 压缩存活的对象
        // ...
        // 清除未被标记的对象
        // ...
    }
}

3. Summary
   The JVM garbage collection algorithm is an important part of the Java application running environment and is responsible for automatically managing memory. This article introduces common garbage collection algorithms, including mark-sweep algorithm, copy algorithm and mark-compression algorithm, and explains the application of the algorithm through specific code examples.

By rationally selecting the garbage collection algorithm, the pause time and memory usage of the application can be effectively reduced, and the performance and availability of the system can be improved. At the same time, understanding the principles and characteristics of various algorithms will help developers optimize and tune the memory management of Java applications.

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