java单例模式怎么实现

Java中单例模式的实现方法有以下几种：

懒汉式（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;}}

懒汉式（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;}}

饿汉式：

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

双重检查锁定（Double-Checked Locking）：

public class Singleton {private volatile static Singleton instance;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：

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

以上是几种常见的单例模式实现方法，每种方法都有各自的优缺点，可以根据具体需求选择适合的实现方法。