使用示例：许多开发者将单例模式视为一种反模式。 因此它在 Java 代码中的使用频率正在逐步减少。
尽管如此， Java 核心程序库中仍有相当多的单例示例：
java.lang.Runtime#getRuntime()java.awt.Desktop#getDesktop()java.lang.System#getSecurityManager()
识别方法：单例可以通过返回相同缓存对象的静态构建方法来识别。
01基础单例（单线程）
实现一个粗糙的单例非常简单。 你仅需隐藏构造函数并实现一个静态的构建方法即可。
Singleton.java: 单例
package refactoring_guru.singleton.example.non_thread_safe; public final class Singleton { private static Singleton instance; public String value; private Singleton(String value) { // The following code emulates slow initialization. try { Thread.sleep(1000); } catch (InterruptedException ex) { ex.printStackTrace(); } this.value = value; } public static Singleton getInstance(String value) { if (instance == null) { instance = new Singleton(value); } return instance; } } DemoSingleThread.java: 客户端代码 package refactoring_guru.singleton.example.non_thread_safe; public class DemoSingleThread { public static void main(String[] args) { System.out.println("If you see the same value, then singleton was reused (yay!)" + "\n" + "If you see different values, then 2 singletons were created (booo!!)" + "\n\n" + "RESULT:" + "\n"); Singleton singleton = Singleton.getInstance("FOO"); Singleton anotherSingleton = Singleton.getInstance("BAR"); System.out.println(singleton.value); System.out.println(anotherSingleton.value); } }
OutputDemoSingleThread.txt: 执行结果
If you see the same value, then singleton was reused (yay!) If you see different values, then 2 singletons were created (booo!!) RESULT: FOO FOO
02基础单例（多线程）
相同的类在多线程环境中会出错。 多线程可能会同时调用构建方法并获取多个单例类的实例。
Singleton.java: 单例
package refactoring_guru.singleton.example.non_thread_safe; public final class Singleton { private static Singleton instance; public String value; private Singleton(String value) { // The following code emulates slow initialization. try { Thread.sleep(1000); } catch (InterruptedException ex) { ex.printStackTrace(); } this.value = value; } public static Singleton getInstance(String value) { if (instance == null) { instance = new Singleton(value); } return instance; } } DemoMultiThread.java: 客户端代码 package refactoring_guru.singleton.example.non_thread_safe; public class DemoMultiThread { public static void main(String[] args) { System.out.println("If you see the same value, then singleton was reused (yay!)" + "\n" + "If you see different values, then 2 singletons were created (booo!!)" + "\n\n" + "RESULT:" + "\n"); Thread threadFoo = new Thread(new ThreadFoo()); Thread threadBar = new Thread(new ThreadBar()); threadFoo.start(); threadBar.start(); } static class ThreadFoo implements Runnable { @Override public void run() { Singleton singleton = Singleton.getInstance("FOO"); System.out.println(singleton.value); } } static class ThreadBar implements Runnable { @Override public void run() { Singleton singleton = Singleton.getInstance("BAR"); System.out.println(singleton.value); } } }
OutputDemoMultiThread.txt: 执行结果
If you see the same value, then singleton was reused (yay!) If you see different values, then 2 singletons were created (booo!!) RESULT: FOO BAR
03采用延迟加载的线程安全单例
为了解决这个问题， 你必须在创建首个单例对象时对线程进行同步。
Singleton.java: 单例
package refactoring_guru.singleton.example.thread_safe; public final class Singleton { // The field must be declared volatile so that double check lock would work // correctly. private static volatile Singleton instance; public String value; private Singleton(String value) { this.value = value; } public static Singleton getInstance(String value) { // The approach taken here is called double-checked locking (DCL). It // exists to prevent race condition between multiple threads that may // attempt to get singleton instance at the same time, creating separate // instances as a result. // // It may seem that having the `result` variable here is completely // pointless. There is, however, a very important caveat when // implementing double-checked locking in Java, which is solved by // introducing this local variable. // // You can read more info DCL issues in Java here: // https://refactoring.guru/java-dcl-issue Singleton result = instance; if (result != null) { return result; } synchronized(Singleton.class) { if (instance == null) { instance = new Singleton(value); } return instance; } } }
DemoMultiThread.java: 客户端代码
package refactoring_guru.singleton.example.thread_safe; public class DemoMultiThread { public static void main(String[] args) { System.out.println("If you see the same value, then singleton was reused (yay!)" + "\n" + "If you see different values, then 2 singletons were created (booo!!)" + "\n\n" + "RESULT:" + "\n"); Thread threadFoo = new Thread(new ThreadFoo()); Thread threadBar = new Thread(new ThreadBar()); threadFoo.start(); threadBar.start(); } static class ThreadFoo implements Runnable { @Override public void run() { Singleton singleton = Singleton.getInstance("FOO"); System.out.println(singleton.value); } } static class ThreadBar implements Runnable { @Override public void run() { Singleton singleton = Singleton.getInstance("BAR"); System.out.println(singleton.value); } } }
OutputDemoMultiThread.txt: 执行结果
If you see the same value, then singleton was reused (yay!) If you see different values, then 2 singletons were created (booo!!) RESULT: BAR BAR
