单例模式源码分析

Runtimejava（饿汉式）

public class Runtime {
    private static Runtime currentRuntime = new Runtime();

    public static Runtime getRuntime() {
        return currentRuntime;
    }

    /** Don't let anyone else instantiate this class */
    private Runtime() {}
    
    //省略一堆方法
}

Desktop.java(容器单例)

public class Desktop {

    private DesktopPeer peer;

    private Desktop() {
        peer = Toolkit.getDefaultToolkit().createDesktopPeer(this);
    }


    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);
        }

        return desktop;
    }
//省略一堆方法

AbstractFactoryBean()

public abstract class AbstractFactoryBean<T>
        implements FactoryBean<T>, BeanClassLoaderAware, BeanFactoryAware, InitializingBean, DisposableBean {
        @Override
    public final T getObject() throws Exception {
        if (isSingleton()) {
            return (this.initialized ? this.singletonInstance : getEarlySingletonInstance());//没实例化，就获取早期的单例对象
        }
        else {
            return createInstance();
        }
    }
    @SuppressWarnings("unchecked")
    private T getEarlySingletonInstance() throws Exception {
        Class<?>[] ifcs = getEarlySingletonInterfaces();
        if (ifcs == null) {
            throw new FactoryBeanNotInitializedException(
                    getClass().getName() + " does not support circular references");
        }
        if (this.earlySingletonInstance == null) {
            this.earlySingletonInstance = (T) Proxy.newProxyInstance(//通过代理对象生成早期单例对象
                    this.beanClassLoader, ifcs, new EarlySingletonInvocationHandler());
        }
        return this.earlySingletonInstance;
    }
    }
    //省略一堆方法

ErrorContext（ThreadLocal）

public class ErrorContext {

  private static final String LINE_SEPARATOR = System.getProperty("line.separator","\n");
  private static final ThreadLocal<ErrorContext> LOCAL = new ThreadLocal<ErrorContext>();

  private ErrorContext stored;
  private String resource;
  private String activity;
  private String object;
  private String message;
  private String sql;
  private Throwable cause;

  private ErrorContext() {
  }

  public static ErrorContext instance() {
    ErrorContext context = LOCAL.get();
    if (context == null) {
      context = new ErrorContext();
      LOCAL.set(context);
    }
    return context;
  }
  //省略一堆方法
  }