The singleton pattern

1. Singleton – Static constant

public class Singleton {

    private static Singleton singleton = new Singleton();

    private Singleton(a) {}

    public static Singleton getInstance(a) {
        returnsingleton; }}Copy the code

2. Singleton pattern – static code block

public class Singleton {

    private static Singleton singleton;

    static {
        singleton = new Singleton();
    }

    private Singleton(a) {}

    public static Singleton getInstance(a) {
        returnsingleton; }}Copy the code

3. Singleton pattern – static inner class

public class Singleton {

    private static class SingletonInner {

        private static Singleton singleton = new Singleton();
    }

    private Singleton(a) {}

    public static Singleton getInstance(a) {
        returnSingletonInner.singleton; }}Copy the code

4. Singleton pattern-synchronous method

public class Singleton {

    private static Singleton singleton;

    private Singleton(a) {}

    synchronized public static Singleton getInstance(a) {
        if (singleton == null) {
            singleton = new Singleton();
        }
        returnsingleton; }}Copy the code

5. Singleton mode – Synchronized code blocks

public class Singleton {

    private static Singleton singleton;

    private Singleton(a) {}

    public static Singleton getInstance(a) {
        synchronized (Singleton.class) {
            if (singleton == null) {
                singleton = newSingleton(); }}returnsingleton; }}Copy the code

6. Singleton mode -DCL double check

public class Singleton {

    volatile private static Singleton singleton;

    private Singleton(a) {}

    public static Singleton getInstance(a) {
        if (singleton == null) {
            synchronized (Singleton.class) {
                if (singleton == null) {
                    singleton = newSingleton(); }}}returnsingleton; }}Copy the code

7. Singleton mode – Enumeration

public enum SingletonEnum {

    SINGLETON;

    private Object singleton;

    SingletonEnum() {
        singleton = new Object();
    }

    public Object getInstance(a) {
        returnsingleton; }}Copy the code

8. Singleton is broken by serialization deserialization

public class Singleton implements Serializable {

    private static final long serialVersionUID = -7084416999763919708L;

    volatile private static Singleton singleton;

    private Singleton(a) {}

    public static Singleton getInstance(a) {
        if (singleton == null) {
            synchronized (Singleton.class) {
                if (singleton == null) {
                    singleton = newSingleton(); }}}return singleton;
    }

// protected Object readResolve() throws ObjectStreamException {
// return singleton;
/ /}
}

public class Test {

    public static void main(String[] args) {
        try (FileOutputStream outputStream = new FileOutputStream(new File("Singleton.txt"));
             ObjectOutputStream oos = new ObjectOutputStream(outputStream)) {
            Singleton singleton1 = Singleton.getInstance();
            System.out.println(singleton1.hashCode());
            oos.writeObject(singleton1);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        try (FileInputStream inputStream = new FileInputStream(new File("Singleton.txt"));
             ObjectInputStream ois = new ObjectInputStream(inputStream)) {
            Singleton singleton2 = (Singleton) ois.readObject();
            System.out.println(singleton2.hashCode());
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch(ClassNotFoundException e) { e.printStackTrace(); }}}Copy the code

• After executing the above code, the Singleton instance is not the same because, by default, serialization creates a new object by calling the no-argument constructor by reflection

• Defining the readResolve() method in the Singleton class and implementing the policy of generating objects prevents Singleton from being broken

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