Java设计模式(1) -- 单例

Singleton

 

英文简单描述
Intent
Ensure a class only has one instance, and provide a global point for accessing to it.

How to

We make it a Singleton class in Java by adding a static getInstance operation and a static instance member.
We must also protect the constructor to prevent accidental instantiation.

Known Cases

Socket

Database Connection

UML

 

通常形式的单例:

public class Singleton { private static Singleton unique = null; public static Singleton getInstance() { if (unique == null) { //Lazy loading unique = new Singleton(); } return unique; } protected String testString; protected Singleton() { testString = "Singleton"; } public void printString() { System.out.println(testString); } }

 

子类化的单例实现:在运行时由配置文件来决定实例化哪个子类

public class Singleton { private static Singleton unique = null; public static Singleton getInstance() { if (unique == null) { //Lazy loading String env = getEnv(); if (env.equals("SubSingleton")) { unique = new SubSingleton(); } else { unique = new Singleton(); } } return unique; } protected String testString; protected Singleton() { testString = "Singleton"; } public void printString() { System.out.println(testString); } private static String getEnv() { try { File file = new File("org//haha//DesignPatterns//singleton//Env.conf"); FileInputStream in = new FileInputStream(file); Properties props = new Properties(); props.load(in); return props.getProperty("classname"); } catch (IOException e) { return ""; } } } public class SubSingleton extends Singleton { protected SubSingleton() { testString = "SubSingleton"; } } public class TestSingleton { public static void main(String[] args) { Singleton singleton = Singleton.getInstance(); singleton.printString(); } } Env.conf classname=aaa