上篇我們學習了最簡單的單例模式，但在有些應用下，我們希望只有在使用到該類的時候才去建立例項，而不是在類載入的時候。

下面我們看到的單例模式實現在多執行緒併發情況下是不安全的。

public class SingletonUnSafe {

	private static SingletonUnSafe instance;

	private SingletonUnSafe() {

	}

	public static SingletonUnSafe getInstance() {
		if (null == instance) {
			instance = new SingletonUnSafe();
		}
		return instance;
	}

}
 

if (null == instance)
{
	instance = new SingletonUnSafe();
}

public class SingletonUnSafe {

	private static SingletonUnSafe instance;

	private SingletonUnSafe() {

	}

	public static synchronized SingletonUnSafe getInstance() {
		if (null == instance) {
			instance = new SingletonUnSafe();
		}
		return instance;
	}

}
 

public class SingletonThreadSafe {

	private volatile static SingletonThreadSafe instance;

	private SingletonThreadSafe() {
		
	}

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

if (null == instance){
	synchronized (SingletonThreadSafe.class) {
	    if (null == instance) {
		    instance = new SingletonThreadSafe();
	    }
	}
}

現在我們已經完成了執行緒安全的單例模式。