Java单例模式的9种实现方式

Java单例模式的9种实现方式

一.饿汉式

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

JDK中，java.lang.Runtime就是典型的使用饿汉式的单例模式

二.懒汉式，线程不安全

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

三.懒汉式，线程安全

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

四.双检锁/双重校验锁（DCL，即 double-checked locking）

JDK 版本：JDK1.5 起

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;  
    }  
}

五.静态内部类

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

六.枚举

JDK 版本：JDK1.5 起

public enum Singleton {
    
    INSTANCE;  
    public void whateverMethod() {
    
    }  
}

七.双检锁/双重校验锁（DCL，即 double-checked locking）(加volatile)

JDK 版本：JDK1.5 起

public class Singleton {
    
	# 使用volatile设置内存屏障，确保线程访问资源可见性。
    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;  
    }  
}

八.使用ThreadLocal实现单例

public class Singleton {
    
	// 用空间换时间
	private static final ThreadLocal<Singleton> tl = new ThreadLocal<Singleton>();
    @Override
    protected Singleton initialValue() {
  
        return new Singleton();
    }

    private Singleton() {
  }

    public static Singleton getInstance() {
  
        return tl.get();
    }
}

九.使用CAS锁实现单例

private static final AtomicReference<Singleton> INSTANCE = new AtomicReference<Singleton>();

    private Singleton() {
  
    }

    public static final Singleton getInstance() {
  
        for (; ; ) {
  
            Singleton current = INSTANCE.get();
            if (current != null) {
  
                return current;
            }

            current = new Singleton();
            if (INSTANCE.compareAndSet(null, current)) {
  
                return current;
            }
        }
    }