Shulou(Shulou.com)06/03 Report--

Singleton mode

Definition: ensures that there is only one instance of a class and provides a global access point to it. The class diagram is as follows:

The code is implemented as follows:

Public class Singleton {/ / uses static variables to record the unique instance private static Singleton instance / * Constructor is privatized to prevent it from being created. Only the constructor * / private Singleton () {} / * * can be used inside the class to instantiate the object and return the object * @ return * / public static Singleton getInstance () {if (instance = = null) {instance = new Singleton () } return instance;} / / other methods}

There will be problems with the above code in the concurrent environment. We will further modify it as follows:

/ * instantiate the object and return it * synchronized locking may affect performance * @ return * / public static synchronized SingletonThread getInstance () {if (instance = = null) {instance = new SingletonThread ();} return instance;}

The above example has a great impact on performance, can we further improve it?

If the performance of getInstance () is not critical to the application, don't do anything, and don't improve the use of "eager" to create an instance without delaying instantiation. The code is as follows: public class Singleton3 {/ / static property, initialized when the class is loaded, JVM guarantees unique private static Singleton3 instance = new Singleton3 (); private Singleton3 () {} public static Singleton3 getInstance () {return instance }} use "double check locking" to reduce the use of synchronization in getInstance (), which is not suitable for previous versions of JDK1.4, as follows Public class Singleton4 {/ / uses static variables to record unique instances. In volatile multithreaded environment, the latest value private volatile static Singleton4 instance is guaranteed. / * Constructors are privatized to prevent creation, and objects can only be instantiated using the constructor * / private Singleton4 () {} / * inside the class. And return this object * @ return * / public static Singleton4 getInstance () {if (instance = = null) {synchronized (Singleton4.class) {if (instance = = null) {instance = new Singleton4 () } return instance;} / / other methods}

Note: when there are multiple class loaders, simple interest may produce multiple oh, this should be noted.

Application scenarios of singleton mode:

An instance object in an application needs to be accessed frequently. There is only one instance per startup in the application. Such as account system, database system.

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