public class Test {
/**
* 单例模式，懒汉式，线程安全
*/
public static class Singleton {
private final static Singleton INSTANCE = new Singleton();
private Singleton() {
}
public static Singleton getInstance() {
return INSTANCE;
}
}
/**
* 单例模式，懒汉式，线程不安全
*/
public static class Singleton2 {
private static Singleton2 instance = null;
private Singleton2() {
}
public static Singleton2 getInstance() {
if (instance == null) {
instance = new Singleton2();
}
return instance;
}
}
/**
* 单例模式，饿汉式，线程安全，多线程环境下效率不高
*/
public static class Singleton3 {
private static Singleton3 instance = null;
private Singleton3() {
}
public static synchronized Singleton3 getInstance() {
if (instance == null) {
instance = new Singleton3();
}
return instance;
}
}
/**
* 单例模式，懒汉式，变种，线程安全
*/
public static class Singleton4 {
private static Singleton4 instance = null;
static {
instance = new Singleton4();
}
private Singleton4() {
}
public static Singleton4 getInstance() {
return instance;
}
}
/**
* 单例模式，使用静态内部类，线程安全（推荐）
*/
public static class Singleton5 {
private final static class SingletonHolder {
private static final Singleton5 INSTANCE = new Singleton5();
}
private static Singleton5 getInstance() {
return SingletonHolder.INSTANCE;
}
}
/**
* 静态内部类，使用枚举方式，线程安全（推荐）
*/
public enum Singleton6 {
INSTANCE;
public void whateverMethod() {
}
}
/**
* 静态内部类，使用双重校验锁，线程安全（推荐）
*/
public static class Singleton7 {
private volatile static Singleton7 instance = null;
private Singleton7() {
}
public static Singleton7 getInstance() {
if (instance == null) {
synchronized (Singleton7.class) {
if (instance == null) {
instance = new Singleton7();
}
}
}
return instance;
}
}
}