单例的7种写法

懒汉模式-线程不安全

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public class Singleton {
private static Singleton instance;
private Singleton (){

}

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

懒汉模式-线程安全

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public class Singleton {
private static Singleton instance;
private Singleton (){

}

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

饿汉模式

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

private Singleton (){

}

public static Singleton getInstance() {
return instance;
}
}

饿汉模式-变种

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public class Singleton {
private Singleton instance = null;

static {
instance = new Singleton();
}

private Singleton (){

}
public static Singleton getInstance() {
return this.instance;
}
}

静态内部类

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

private Singleton (){

}

public static final Singleton getInstance() {
return SingletonHolder.INSTANCE;
}
}

双重校验锁

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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;
}
}

枚举

12345

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