2. Singleton pattern of Java design pattern

xiaocantian 2020-11-09 17:44:33
singleton pattern java design pattern


  • Design pattern is a useful experience in the face of similar software engineering design problems , Patterns are not code , It's a general solution to some kind of problem
  • The essence of design patterns is to improve the maintainability of software , Generality and expansibility , And reduce the complexity of the software
  • java Design patterns are 23 Kind of , Divided into three categories
    • Create pattern : The singleton pattern 、 Abstract factory pattern 、 Archetypal model 、 Builder pattern 、 Factory mode
    • Structural mode : Adapter pattern 、 Bridging mode 、 Decoration mode 、 Portfolio model 、 Appearance mode 、 The flyweight pattern 、 The proxy pattern
    • Behavioral patterns : Template method pattern 、 Command mode 、 Visitor mode 、 Iterator pattern 、 Observer mode 、 Intermediary model 、 Memo mode 、 Interpreter mode 、 The state pattern 、 The strategy pattern 、 The chain of responsibility model

1、 The singleton pattern

Basic introduction :

​ Take certain methods to ensure that in the whole software system , There can only be one instance object for a class , And this class only provides a static method to get its object instance , share 8 Ways of planting

Code implementation :
  1. /**
    * Hungry Chinese style ( static const )
    * Advantages and disadvantages
    * Written in simple , Complete the instantiation when the class is loaded , The thread safety problem is avoided
    * It doesn't delay loading , If the object has not been used , It causes a waste of memory
    * This way is based on classloder The mechanism avoids the synchronization problem of multithreading , however ,instance Instantiate at class load time , In singleton mode, most of them call getInstance Method
    * But there are many reasons for class loading , So it's not certain there are other ways ( Or other static methods ) Causes the class to load , This time initialization instance It didn't reach lazy loading The effect of
    *
    */
    class Singleton {
    private static Singleton singleton = new Singleton();
    /**
    * Privatized structure Prevent external creation
    */
    private Singleton() {}
    public static Singleton getInstance() {
    return singleton;
    }
    }
    
  2. /**
    * Hungry Chinese style ( Static code block )
    * Advantages and disadvantages :
    * And the hungry Han style ( Static variables ) The same as , Just put the instantiation in a static code block
    */
    class Singleton {
    private static Singleton singleton;
    private Singleton() {}
    static {
    singleton = new Singleton();
    }
    public static Singleton getInstance() {
    return singleton;
    }
    }
    
  3. /**
    * Slacker type ( Thread unsafe )
    * Advantages and disadvantages :
    * It has the effect of delayed loading , But in a multithreaded environment, there are thread safety issues
    * for example : More than one thread enters at the same time if Sentence block It is not recommended to use
    */
    class Singleton {
    private static Singleton singleton;
    private Singleton() {}
    /**
    * Use it to create instance objects
    * @return
    */
    public static Singleton getInstance() {
    if (null == singleton) {
    singleton = new Singleton();
    }
    return singleton;
    }
    }
    
  4. /**
    * Slacker type ( Synchronization method )
    * Advantages and disadvantages :
    * Solved thread safety problems , But it's too inefficient , The code that creates the instance only needs to be executed once It is not recommended to use
    */
    class Singleton {
    private static Singleton singleton;
    private Singleton() {}
    /**
    * Using synchronous methods to ensure thread safety
    * @return
    */
    public synchronized static Singleton getInstance() {
    if (null == singleton) {
    singleton = new Singleton();
    // Be careful : The synchronization code block here is completely redundant It can't solve the problem of thread safety
    // More than one thread is counted in at the same time if Sentence block , Multiple instances will still be created
    synchronized (Singleton.class) {
    singleton = new Singleton();
    }
    }
    return singleton;
    }
    }
    
  5. /**
    * Slacker type ( Synchronization code block )
    * Advantages and disadvantages :
    * It doesn't solve the thread safety problem , It is not recommended to use
    */
    class Singleton {
    private static Singleton singleton;
    private Singleton() {}
    public static Singleton getInstance() {
    if (null == singleton) {
    // Be careful : The synchronization code block here is completely redundant It can't solve the problem of thread safety
    // More than one thread is counted in at the same time if Sentence block , Multiple instances will still be created
    synchronized (Singleton.class) {
    singleton = new Singleton();
    }
    }
    return singleton;
    }
    }
    
  6. /**
    * Slacker type ( Double check )
    * Advantages and disadvantages :
    * Double-Check Solve the thread safety problem of multithreading , The instance creation method was executed only once , And it can delay the loading
    * First floor if Improved efficiency , This method is recommended
    */
    class Singleton {
    private static volatile Singleton singleton;
    private Singleton() {}
    public static Singleton getInstance() {
    if (null == singleton) {
    // Even if multiple threads enter the same time if sentence , The instantiated code will only be executed once
    synchronized (Singleton.class) {
    if (null == singleton) {
    singleton = new Singleton();
    }
    }
    }
    return singleton;
    }
    }
    
  7. /**
    * Static inner class
    * Advantages and disadvantages :
    * In this way, the class loading mechanism is adopted to ensure that there is only one thread when initializing an instance
    * Static inner class mode is in Singleton Classes are not instantiated immediately when they are loaded , But when you need to instantiate , call getInstance Method , Will be loaded SingletonInstance class , To complete the Singleton Instantiation
    * The static properties of a class are initialized only when the class is first loaded , So here JVM Help us to ensure the safety of threads , When class initializes , Other threads are not accessible
    * Realize thread safety , Delay loading , Efficient , Recommended
    */
    class Singleton {
    private static volatile Singleton singleton;
    private Singleton() {}
    /**
    * Static inner class
    */
    private static class SingletonInstance {
    private static final Singleton INSTANCE = new Singleton();
    }
    public static Singleton getInstance() {
    return SingletonInstance.INSTANCE;
    }
    }
    
  8. /**
    * enumeration
    * Advantages and disadvantages :
    * Not only can multithreading synchronization be avoided , It also prevents deserialization from recreating new objects Recommended
    */
    enum Singleton {
    INSTANCE;
    }
    
jdk Source code Runtime Class uses the hungry Chinese style
  • public class Runtime {
    private static Runtime currentRuntime = new Runtime();
    /**
    * Returns the runtime object associated with the current Java application.
    * Most of the methods of class <code>Runtime</code> are instance
    * methods and must be invoked with respect to the current runtime object.
    *
    * @return the <code>Runtime</code> object associated with the current
    * Java application.
    */
    public static Runtime getRuntime() {
    return currentRuntime;
    }
    /** Don't let anyone else instantiate this class */
    private Runtime() {}
    ......
    }
    
matters needing attention :
  • Singleton mode ensures that there is only one object in the system memory , Save system resources , For some objects that need to be destroyed frequently , Using singleton mode can improve system performance
  • When you want to instantiate a singleton class , You have to use the corresponding method to get the object , Instead of using new
  • Scenarios used : Objects that need to be created and destroyed frequently 、 It takes too much time or resources to create an object ( namely : Heavyweight objects ), But it's something that we use a lot 、 Utility class object 、 Objects that frequently access databases or files ( For example, data source 、session The factory etc. )
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