设计模式课程 设计模式精讲 8-11 单例模式源码解析（jdk+spring+mybaties）

1　　　　源码解析

1.1　　单例解析1

1.2　　单例解析2（容器单例）

1.3　　单例解析3

1.4　　单例解析4

1　　　　源码解析

1.1　　单例解析1

java.lang.Runtime

/**
 *    饿汉式加载，初始化的时候，就已经new出了对象
 */ 
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;
    }

1.2　　单例解析2（容器单例）

java.awt.Desktop(cs)

 /**
     * Returns the <code>Desktop</code> instance of the current
     * browser context.  On some platforms the Desktop API may not be
     * supported; use the {@link #isDesktopSupported} method to
     * determine if the current desktop is supported.
     * @return the Desktop instance of the current browser context
     * @throws HeadlessException if {@link
     * GraphicsEnvironment#isHeadless()} returns {@code true}
     * @throws UnsupportedOperationException if this class is not
     * supported on the current platform
     * @see #isDesktopSupported()
     * @see java.awt.GraphicsEnvironment#isHeadless
     */

    /*
     *    同步锁，context取对象，如果该对象为为null，new出新的对象，然后放入context
     */
    public static synchronized Desktop getDesktop(){
        if (GraphicsEnvironment.isHeadless()) throw new HeadlessException();
        if (!Desktop.isDesktopSupported()) {
            throw new UnsupportedOperationException("Desktop API is not " +
                                                    "supported on the current platform");
        }

        sun.awt.AppContext context = sun.awt.AppContext.getAppContext();
        Desktop desktop = (Desktop)context.get(Desktop.class);

        if (desktop == null) {
            desktop = new Desktop();
            context.put(Desktop.class, desktop);
        }


    /**
     *  context put的时候加上同步锁，可以避免多线程put异常
     */
    public Object put(Object var1, Object var2) {
        HashMap var3 = this.table;
        synchronized(this.table) {
            MostRecentKeyValue var4 = this.mostRecentKeyValue;
            if (var4 != null && var4.key == var1) {
                var4.value = var2;
            }

            return this.table.put(var1, var2);
        }
    }

1.3　　单例解析3（Spring框架获取单例对象）

spring中的单例是bean作用域中的一个，作用域在每个应用程序的上下文中只创建一个我们设置属性的实例，

和我们的单例的区别是：spring将实例的数量限制的作用域在整个应用程序的上下文，而java应用程序中，是将类加载器的数量限制在给定的类加载器的整个空间里。

所以说，在spring中启动多个容器的时候，每个容器即使是单例的，都可以拿到这个对象。

  public final T getObject() throws Exception {
        if (this.isSingleton()) {
            return this.initialized ? this.singletonInstance : this.getEarlySingletonInstance();
        } else {
            return this.createInstance();
        }
    }

/*
 *    如果被初始化，获取早期的单例对象
 * 
 */
//通过代理去拿新对象
    private T getEarlySingletonInstance() throws Exception {
        Class<?>[] ifcs = this.getEarlySingletonInterfaces();
        if (ifcs == null) {
            throw new FactoryBeanNotInitializedException(this.getClass().getName() + " does not support circular references");
        } else {
            if (this.earlySingletonInstance == null) {
                this.earlySingletonInstance = Proxy.newProxyInstance(this.beanClassLoader, ifcs, new AbstractFactoryBean.EarlySingletonInvocationHandler());
            }

            return this.earlySingletonInstance;
        }
    }

1.4　　单例解析4（基于threadLocal的线程案例）（mybaties获取单例对象）

mybaties上下文保证了每个线程各自的数据，每个线程自己的上下文，自己保存好

  private static final ThreadLocal<ErrorContext> LOCAL = new ThreadLocal<ErrorContext>();
  
 private ErrorContext() {
  }

  public static ErrorContext instance() {
    ErrorContext context = LOCAL.get();
if (context == null) {
      context = new ErrorContext();
      LOCAL.set(context);
    }
    return context;
  }