设计模式 - 懒加载

Posted 2021-04-25 IT之心

类别

其他

意图

懒加载是一种常见的模式用于延迟初始化一个对象,直到需要的时候。 它可以有助于应用的运作效率适当使用。

适用范围

• 加载对象是昂贵的，或者根本不需要加载对象

主要代码

public class Heavy {

    private static final Logger LOGGER = LoggerFactory.getLogger(Heavy.class);

    /**
     * Constructor
     */
    public Heavy() {
        LOGGER.info("Creating Heavy ...");
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            LOGGER.error("Exception caught.", e);
        }
        LOGGER.info("... Heavy created");
    }

}

public class Java8Holder {

    private static final Logger LOGGER = LoggerFactory.getLogger(Java8Holder.class);

    private Supplier<Heavy> heavy = this::createAndCacheHeavy;

    public Java8Holder() {
        LOGGER.info("Java8Holder created");
    }

    public Heavy getHeavy() {
        return heavy.get();
    }

    private synchronized Heavy createAndCacheHeavy(){

        class HeavyFactory implements Supplier<Heavy>{
            private final Heavy heavyInstance = new Heavy();

            @Override
            public Heavy get() {
                return heavyInstance;
            }
        }

        if (!HeavyFactory.class.isInstance(heavy)) {
            heavy = new HeavyFactory();
        }
        return heavy.get();
    }
}

使用

    // The most efficient lazy loader utilizing Java 8 features
    Java8Holder java8Holder = new Java8Holder();
    Heavy next = java8Holder.getHeavy();

引用

• Pattern Oriented Software Architecture Vol I-V

• Design patterns implemented in Java