一·单例模式

保证内存中只有一个实例

方法1：用private修饰构造方法

package singleton;

public class Mgr01 {
    private static final Mgr01 INSTANCE = new Mgr01();

    private Mgr01(){};

    public static Mgr01 getInstance(){return INSTANCE;};

    public void m(){
        System.out.println("m");
    }

    public static void main(String[] args) {
        Mgr01 m1 = Mgr01.getInstance();
        Mgr01 m2 = Mgr01.getInstance();
        System.out.println(m1 == m2);

    }
}

方法2：定义静态语句块

package singleton;

public class Mgr01 {
    private static final Mgr01 INSTANCE;

    static {
        INSTANCE = new Mgr01();
    }

    private Mgr01(){};

    public static Mgr01 getInstance(){return INSTANCE;};

    public void m(){
        System.out.println("m");
    }

    public static void main(String[] args) {
        Mgr01 m1 = Mgr01.getInstance();
        Mgr01 m2 = Mgr01.getInstance();
        System.out.println(m1 == m2);

    }
}

　方法3：懒汉式，线程不安全，使用时需要加锁，会降低效率

package singleton;

public class Mgr01 {
    private static Mgr01 INSTANCE;

    private Mgr01(){};

    public static Mgr01 getInstance(){
        if(INSTANCE == null){
            INSTANCE = new Mgr01();
        }
        return INSTANCE;
    };

    public void m(){
        System.out.println("m");
    }

    public static void main(String[] args) {
        Mgr01 m1 = Mgr01.getInstance();
        Mgr01 m2 = Mgr01.getInstance();
        System.out.println(m1 == m2);

    }
}

 方法4： 从静态内部类获取实例，相比方法1，只有在调用getInstance的时候才会加载静态内部类，即实现了懒加载，又保证线程安全

package singleton;

public class Mgr01 {

    private Mgr01(){};

    private static class Mgr01Holder{
        private static final Mgr01 INSTANCE = new Mgr01();
    }

    public static synchronized Mgr01 getInstance(){
        return Mgr01Holder.INSTANCE;
    };

    public void m(){
        System.out.println("m");
    }

    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            new Thread(() -> System.out.println(Mgr01.getInstance().hashCode())).start();
        }

    }
}

方法5：枚举单例

package singleton;

public enum Mgr08 {
    INSTANCE;

    public void m(){}

    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            new Thread(() -> System.out.println(Mgr08.INSTANCE.hashCode())).start();
        }
    }
}