Summary of five ways to implement singleton pattern in Java

How to implement a single case

1. The constructor needs to be privatized

2. Provide a private static variable

3. Expose a common interface to obtain singleton objects

Two issues to consider

1. Whether lazy loading is supported

2. Is it thread safe?

1. Hungry Man Style

public class EagerSingleton {
    private static final EagerSingleton INSTANCE = new EagerSingleton();

    private EagerSingleton(){}

    public EagerSingleton getInstance(){
        return INSTANCE;
    }

}

Lazy loading is not supported

Thread safety

2. Lazy style

public class LazySingleton {
    private static LazySingleton INSTANCE;

    private LazySingleton() {
    }

    public static LazySingleton getInstance() {
        if (INSTANCE == null) {
            INSTANCE = new LazySingleton();
        }
        return INSTANCE;
    }
}

Support lazy loading

Threads are not safe

public class LazySingleton {
    private static LazySingleton INSTANCE;

    private LazySingleton() {
    }

    public static synchronized LazySingleton getInstance() {
        if (INSTANCE == null) {
            INSTANCE = new LazySingleton();
        }
        return INSTANCE;
    }
}

Support lazy loading

Thread safety

3. Double check lock

public class DoubleCheckSingleton {
    private static DoubleCheckSingleton INSTANCE;

    private DoubleCheckSingleton() {
    }

    public DoubleCheckSingleton getInstance() {
        if (INSTANCE == null) {
            synchronized () {
                if (INSTANCE == null) {
                    INSTANCE = new DoubleCheckSingleton();
                }
            }
        }
        return INSTANCE;
    }
}

Support lazy loading

Thread safety

4. Static internal class

public class InnerSingleton {
    private InnerSingleton() {
    }

    public static InnerSingleton getInstance() {
        return ;
    }

    private static class Singleton {
        private static final InnerSingleton INSTANCE = new InnerSingleton();
    }
}

Support lazy loading

Thread safety

5. Enumeration

public enum EnumSingleton {
    INSTANCE；
}

Summarize

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