Five forms of JVM garbage collection mechanism: detailed interpretation and comparison

Five forms of JVM garbage collection mechanism: detailed interpretation and comparison

Abstract: JVM garbage collection (Garbage Collection, referred to as GC) is one of the core features of the Java language One, because it effectively frees up memory that is no longer in use while the program is running. This article will explain in detail the five forms of JVM garbage collection mechanism and compare the advantages and disadvantages between them. At the same time, we will also provide specific code examples to help readers better understand these garbage collection mechanisms.

1. Introduction
JVM is the abbreviation of Java Virtual Machine, which is the running environment of Java programs. In a Java program, when an object is created in memory, a corresponding mechanism is needed to reclaim the memory space it occupies. This is the task of garbage collection.

2. Mark-Sweep Algorithm (Mark-Sweep)
The mark-sweep algorithm is one of the earliest and most basic garbage collection algorithms. Its principle is simple: first, starting from the root node, all reachable objects are marked; then, unmarked objects are cleared.

Sample code:

public class MarkSweep {
    private boolean marked;
    
    public void setMarked(boolean marked) {
        this.marked = marked;
    }
    
    public boolean isMarked() {
        return marked;
    }
}

public class GC {
    public static void main(String[] args) {
        MarkSweep object1 = new MarkSweep();
        MarkSweep object2 = new MarkSweep();
        
        object1.setMarked(true);
        
        System.gc(); // 垃圾回收
        
        if (object1.isMarked()) {
            System.out.println("object1 is reachable");
        } else {
            System.out.println("object1 is garbage");
        }
        
        if (object2.isMarked()) {
            System.out.println("object2 is reachable");
        } else {
            System.out.println("object2 is garbage");
        }
    }
}

3. Copying algorithm (Copying)
The copying algorithm uses a different strategy to solve the garbage collection problem. It divides the available memory into two blocks and only uses one block at a time. When a piece of memory is used up, copy the surviving objects to another piece of memory, and then clear all objects in the current memory.

Sample code:

public class Copying {
    private boolean marked;
    
    public void setMarked(boolean marked) {
        this.marked = marked;
    }
    
    public boolean isMarked() {
        return marked;
    }
}

public class GC {
    public static void main(String[] args) {
        Copying object1 = new Copying();
        Copying object2 = new Copying();
        
        object1.setMarked(true);
        
        System.gc(); // 垃圾回收
        
        if (object1.isMarked()) {
            System.out.println("object1 is reachable");
        } else {
            System.out.println("object1 is garbage");
        }
        
        if (object2.isMarked()) {
            System.out.println("object2 is reachable");
        } else {
            System.out.println("object2 is garbage");
        }
    }
}

4. Mark-Compact algorithm (Mark-Compact)
The mark-compression algorithm is a garbage collection algorithm that combines the mark-sweep algorithm and the copy algorithm. . It first marks live objects, then moves them to one end, and then clears other objects.

Sample code:

public class MarkCompact {
    private boolean marked;
    
    public void setMarked(boolean marked) {
        this.marked = marked;
    }
    
    public boolean isMarked() {
        return marked;
    }
}

public class GC {
    public static void main(String[] args) {
        MarkCompact object1 = new MarkCompact();
        MarkCompact object2 = new MarkCompact();
        
        object1.setMarked(true);
        
        System.gc(); // 垃圾回收
        
        if (object1.isMarked()) {
            System.out.println("object1 is reachable");
        } else {
            System.out.println("object1 is garbage");
        }
        
        if (object2.isMarked()) {
            System.out.println("object2 is reachable");
        } else {
            System.out.println("object2 is garbage");
        }
    }
}

5. Generational recycling algorithm (Generational)
The generational recycling algorithm uses a more targeted strategy to allocate memory according to the life cycle of the object. Divided into different generations. Normally, newly created objects are allocated to the new generation, and objects that survive multiple GCs are moved to the old generation.

Sample code:

public class Generational {
    private boolean marked;
    
    public void setMarked(boolean marked) {
        this.marked = marked;
    }
    
    public boolean isMarked() {
        return marked;
    }
}

public class GC {
    public static void main(String[] args) {
        Generational object1 = new Generational();
        Generational object2 = new Generational();
        
        object1.setMarked(true);
        
        System.gc(); // 垃圾回收
        
        if (object1.isMarked()) {
            System.out.println("object1 is reachable");
        } else {
            System.out.println("object1 is garbage");
        }
        
        if (object2.isMarked()) {
            System.out.println("object2 is reachable");
        } else {
            System.out.println("object2 is garbage");
        }
    }
}

6. Evaluation and comparison

1. Mark-clearing algorithm is the most basic, but has low efficiency and will produce memory fragmentation.
2. The copy algorithm is simple and efficient, but it can only utilize half of the memory space.
3. Mark - The compression algorithm combines the advantages of the first two algorithms, but requires moving objects and is slightly less efficient.
4. The generational recycling algorithm divides generations according to the life cycle of the object, which can achieve more targeted recycling, but it will increase the complexity of the system.
5. Different garbage collection algorithms are suitable for different application scenarios, and it is very important to choose the appropriate algorithm.

Conclusion:
There are five forms of JVM garbage collection mechanism, each with its own advantages and disadvantages. Choosing an appropriate recycling algorithm requires trade-offs based on specific application scenarios and requirements. This article provides detailed explanations and code examples, hoping to help readers better understand and apply these garbage collection mechanisms.

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