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）：
public class Singleton {
private static Singleton instance;
private Singleton (){}
public static synchronized Singleton getInstance() {
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（普通DCL）：
public class Singleton {
private static Singleton singleton;
private Singleton (){}
public static Singleton getSingleton() {
if (singleton == null) {
synchronized (Singleton.class) {
if (singleton == null) {
singleton = new Singleton();
}
}
}
return singleton;
}
}
第六种（DCL）：
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;
}
}
第七种（DCL）：
public class Singleton {
private static Singleton singleton;
private Singleton (){}
public static Singleton getSingleton() {
if (singleton == null) {
synchronized (Singleton.class) {
if (singleton == null) {
Singleton tmp = new Singleton();
singleton = tmp;
}
}
}
return singleton;
}
}
第八种：
public class Singleton {
private static class SingletonHolder{
static final Singleton INSTANCE = new Singleton();
}
public static Singleton getSingleton() {
return SingletomHolder.INSTANCE;
}
}
}