1,提前初始化

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

public static Singleton getInstance() {
return instance;
}
}

2.雙重檢查鎖定 + volatile。

public class Singleton {
private static volatile Singleton instance;

private Singleton() {
}

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

3.延遲初始化占位類(lèi)模式。

public class Singleton {
private static class InstanceHolder {
public static Singleton instance = new Singleton();
}

private Singleton() {
}

public static Singleton getInstance() {
return InstanceHolder.instance;
}
}