1. The first (lazy, thread unsafe)

public class Singleton { private static Singleton instance; private Singleton() { } public static Singleton getInstance() { if (instance == null) { instance = new Singleton(); } return instance; }}Copy the code

2. The second (lazy, thread-safe)

public class Singleton { private static Singleton instance; private Singleton() { } public static synchronized Singleton getInstance() { if (instance == null) { instance = new Singleton(); } return instance; }}Copy the code

3. (Hungry)

public class Singleton { private static Singleton instance = new Singleton(); private Singleton() { } public static Singleton getInstance() { return instance; }}Copy the code

4. The fourth type (Hungry, variant)

public class Singleton { private static Singleton instance = null; static { instance = new Singleton(); } private Singleton() { } public static Singleton getInstance() { return instance; }}Copy the code

5. Fifth (Static inner Class)

public class Singleton { private static class SingletonHolder { private static final Singleton INSTANCE = new Singleton(); } private Singleton() { } public static final Singleton getInstance() { return SingletonHolder.INSTANCE; }}Copy the code

6. (enumeration)

public enum Singleton {
    INSTANCE;

    public void whateverMethod() {
    }
}
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7. Seventh type (double check lock)

public class Singleton {
    private volatile static Singleton singleton;

    private Singleton() {
    }

    public static Singleton getSingleton() {
        if (singleton == null) {
            synchronized (Singleton.class) {
                if (singleton == null) {
                    singleton = new Singleton();
                }
            }
        }
        return singleton;
    }
} 
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