单例模式在JDK 应用

• ctrl + n，搜索Runtime

public class Runtime {
    private static final Runtime currentRuntime = new Runtime();
 
    private static Version version;
 
    /**
     * Returns the runtime object associated with the current Java application.
     * Most of the methods of class {@code Runtime} are instance
     * methods and must be invoked with respect to the current runtime object.
     *
     * @return  the {@code Runtime} object associated with the current
     *          Java application.
     */
    public static Runtime getRuntime() {
        return currentRuntime;
    }
 
    /** Don't let anyone else instantiate this class */
    private Runtime() {}
 
    /**
     * Terminates the currently running Java virtual machine by initiating its
     * shutdown sequence.  This method never returns normally.  The argument
     * serves as a status code; by convention, a nonzero status code indicates
     * abnormal termination.
     *
     * <p> All registered {@linkplain #addShutdownHook shutdown hooks}, if any,
     * are started in some unspecified order and allowed to run concurrently
     * until they finish.  Once this is done the virtual machine
     * {@linkplain #halt halts}.
     *
     * <p> If this method is invoked after all shutdown hooks have already
     * been run and the status is nonzero then this method halts the
     * virtual machine with the given status code. Otherwise, this method
     * blocks indefinitely.
     *
     * <p> The {@link System#exit(int) System.exit} method is the
     * conventional and convenient means of invoking this method.
     *
     * @param  status
     *         Termination status.  By convention, a nonzero status code
     *         indicates abnormal termination.
     *
     * @throws SecurityException
     *         If a security manager is present and its
     *         {@link SecurityManager#checkExit checkExit} method does not permit
     *         exiting with the specified status
     *
     * @see java.lang.SecurityException
     * @see java.lang.SecurityManager#checkExit(int)
     * @see #addShutdownHook
     * @see #removeShutdownHook
     * @see #halt(int)
     */
    public void exit(int status) {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkExit(status);
        }
        Shutdown.exit(status);
    }

• 注意事项和细节

1) 单例模式保证了 系统内存中该类只存在一个对象，节省了系统资源，对于一些需要频繁创建销毁的对象，使用单例模式可以提高系统性能
2) 当想实例化一个单例类的时候，必须要记住使用相应的获取对象的方法，而不是使用new
3) 单例模式使用的场景：需要频繁的进行创建和销毁的对象、创建对象时耗时过多或耗费资源过多(即：重量级对象)，但又经常用到的对象、工具类对象、频繁访问数据库或文件的对象(比如数据源、session工厂等)