Java中的单例模式

第一种（懒汉，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;
}
}

第二种（懒汉，线程安全）

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

第三种（饿汉）

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

第四种（饿汉，变种）

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

第五种（静态内部类）

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

第六种（枚举）

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

第七中（双重校验锁）

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;
}
}