一、初始化单例类时即创建单例
- 饿汉式：（线程安全）
public class Singleton {
private static Singleton instance = new Singleton();
private Singleton() {
}
public static Singleton getInstance() {
return instance;
}
}
- 枚举类型：（线程安全）
public enum Singleton {
INSTANCE;
}
二、按需、延迟创建单例
- 懒汉式：（线程不安全）
public class Singleton {
private static Singleton instance = null;
private Singleton() {
}
public static Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
}
- 懒汉式（同步锁）：（线程安全）
public class Singleton {
private static Singleton instance = null;
private Singleton() {
}
public static synchronized Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
}
public class Singleton {
private static Singleton instance = null;
private Singleton() {
}
public static Singleton getInstance() {
synchronized (Singleton.class) {
if (instance == null) {
instance = new Singleton();
}
}
return instance;
}
}
- 懒汉式（双重校验锁）：（线程安全）
public class Singleton {
private static Singleton instance = null;
private Singleton() {
}
public static Singleton getInstance() {
if (instance == null) {
synchronized (Singleton.class) {
if (instance == null) {
instance = new Singleton();
}
}
}
return instance;
}
}
- 静态内部类：（线程安全）【推荐使用】
public class Singleton {
private static class Singleton2 {
private static Singleton instance = new Singleton();
}
private Singleton() {
}
public static Singleton getInstance() {
return Singleton2.instance;
}
}
单例模式的实现方式对比.png
