java-單例多例

一：單例

public class SingleObject {
private static SingleObject instance;

public SingleObject(){}

public static SingleObject getInstance(){

if(instance == null){

instance = new SingleObject();

}

return instance;
}

}
//缺點，在多線程狀態下，多個線程同時調用getInstance()會產生多個對象，不推薦




public class SingleObject {
private static SingleObject instance;

public SingleObject(){}

public static synchronized SingleObject getInstance(){

if(instance == null){

instance = new SingleObject();

}

return instance;

}
}
//在getInstances（）加上鎖就可以了，但是隻有第一次西藥new 對象，鎖有性能損耗。


public class SingleObject {

private static SingleObject instance = new SingleObject();


public SingleObject(){}

public static SingleObject getInstance(){

return instance;

}
}
//這種方式還是比較好的




public class SingleObject {

private volatile static SingleObject instance;

public SingleObject(){}

public static SingleObject getInstance(){

if(instance == null){

synchronized (SingleObject.class){

if(instance == null){

instance = new SingleObject();

}
}
}
return instance;

}
}
//雙重檢查也是可以的