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：

1. 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;
}
}

2. 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;
}
}

3. 饿汉式：

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

4. 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;
}
}

5. 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。