java單例模式怎么實現

Java中單例模式的實現方法有以下幾種：

1. 懶漢式（線程不安全）：

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

2. 懶漢式（線程安全）：

public class Singleton {
private static Singleton instance;
private Singleton() {}
public static synchronized Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
}

3. 餓漢式：

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

4. 雙重檢查鎖定（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. 靜態內部類：

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

以上是幾種常見的單例模式實現方法，每種方法都有各自的優缺點，可以根據具體需求選擇適合的實現方法。