java单例模式怎样实现

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. 两重检查锁定（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势和劣势，可以根据具体需求选择合适的实现方法。