In-depth study of JVM garbage collection algorithms: detailed description of common algorithms

In-depth understanding of JVM garbage collection algorithm: several common discussions, requiring specific code examples

Overview:
JVM (Java Virtual Machine) is a Java program running A virtual machine responsible for interpreting and executing Java bytecode files. The JVM garbage collection algorithm is an important part of managing memory. It is responsible for automatically reclaiming memory space that is no longer used to improve program performance and resource utilization. In this article, we will take an in-depth look at several common JVM garbage collection algorithms and provide specific code examples.

1. Mark and Sweep Algorithm (Mark and Sweep)
The mark-sweep algorithm is one of the earliest and most basic garbage collection algorithms. Its implementation idea is to start from the root node (usually a global variable or a reference in a stack frame), recursively traverse the entire object graph, mark all active objects, and then clear unmarked objects. The following is a code example of the mark-clear algorithm:

class GCObject {
    private boolean marked = false;
    // ...
}

class GarbageCollector {
    public static void mark(GCObject object) {
        if (object.isMarked()) {
            return;
        }
        object.setMarked(true);
        // 标记相邻引用的对象
    }
    
    public static void sweep(List<GCObject> objects) {
        for (GCObject object : objects) {
            if (!object.isMarked()) {
                objects.remove(object);
            } else {
                object.setMarked(false);
            }
        }
    }
    
    public static void main(String[] args) {
        // 创建对象并设置引用
        GCObject object1 = new GCObject();
        GCObject object2 = new GCObject();
        object1.setReference(object2);

        // 执行垃圾回收
        List<GCObject> objects = new ArrayList<>();
        objects.add(object1);
        objects.add(object2);
        mark(object1);
        mark(object2);
        sweep(objects);
    }
}

The advantage of the mark-clear algorithm is that it can accurately recycle memory that is no longer used, but it has two main disadvantages: First, a large amount of memory will be left after recycling Discontinuous memory fragmentation leads to low memory utilization; second, the marking and clearing process requires a large amount of computing resources.

2. Copying algorithm (Copying)
The copying algorithm is a garbage collection algorithm proposed to solve the memory fragmentation problem caused by the mark-clear algorithm. The copy algorithm divides the memory space into two areas: From area and To area. When the From area is full, copy the active objects to the To area and clear all unreplicated objects in the From area. The following is a code example of the copy algorithm:

class GCObject {
    // ...
}

class GarbageCollector {
    public static void copy(List<GCObject> objects, int sizeFrom, int sizeTo) {
        List<GCObject> newObjects = new ArrayList<>();
        for (GCObject object : objects) {
            GCObject newObject = object.copyTo(sizeTo);
            newObjects.add(newObject);
        }
        objects.clear();
        objects.addAll(newObjects);
    }
    
    public static void main(String[] args) {
        // 创建对象并设置引用
        GCObject object1 = new GCObject();
        GCObject object2 = new GCObject();
        object1.setReference(object2);

        // 执行垃圾回收
        List<GCObject> objects = new ArrayList<>();
        objects.add(object1);
        objects.add(object2);
        copy(objects, objects.size(), objects.size() * 2);
    }
}

The advantage of the copy algorithm is that it eliminates memory fragmentation and improves memory utilization, but its disadvantage is that it requires a continuous area of ​​the same size as the memory space for copying Object, thus wasting half of the memory space.

3. Mark-Compact Algorithm (Mark and Compact)
The Mark-Compact algorithm is an improved version of the Mark-Clear algorithm, and its main goal is to eliminate memory fragmentation. The mark-compact algorithm first marks active objects and moves them to one end, and then clears the remaining unmarked memory space. The following is a code example of the mark-compact algorithm:

class GCObject {
    private boolean marked = false;
    // ...
}

class GarbageCollector {
    public static void mark(GCObject object) {
        if (object.isMarked()) {
            return;
        }
        object.setMarked(true);
        // 标记相邻引用的对象
    }
    
    public static void compact(List<GCObject> objects) {
        int index = 0;
        for (GCObject object : objects) {
            if (object.isMarked()) {
                swap(objects, index++);
            }
        }
        for (int i = objects.size() - 1; i >= index; i--) {
            objects.remove(i);
        }
    }
    
    public static void swap(List<GCObject> objects, int index) {
        // 交换对象位置
    }
    
    public static void main(String[] args) {
        // 创建对象并设置引用
        GCObject object1 = new GCObject();
        GCObject object2 = new GCObject();
        object1.setReference(object2);

        // 执行垃圾回收
        List<GCObject> objects = new ArrayList<>();
        objects.add(object1);
        objects.add(object2);
        mark(object1);
        mark(object2);
        compact(objects);
    }
}

The advantage of the mark-compact algorithm is that it eliminates memory fragmentation, but its disadvantage is that it requires additional processing steps to move live objects, thereby increasing the complexity of the algorithm sex and overhead.

Summary:
This article provides an in-depth understanding of several common JVM garbage collection algorithms and provides specific code examples. Each algorithm has its advantages and disadvantages, and the appropriate garbage collection algorithm should be selected according to the specific application scenario. I hope that readers can have a deeper understanding of the JVM garbage collection algorithm through the introduction of this article, and can apply it in actual development.

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