动态改变执行方法

题目：有一个接口Processor，分别被ServiceA和ServiceB实现。

interface Processor {
    void process();
}

class ServiceA implements Processor {
    @Override
    public void process() {
        System.out.println("run ServiceA...");
    }
}

class ServiceB implements Processor {
    @Override
    public void process() {
        System.out.println("run ServiceB...");
    }
}

要求前10次由ServiceA运行，超过10次后自动切换成ServiceB运行。

代码如下：

interface Processor {
    void process();
}

class ServiceA implements Processor {
    @Override
    public void process() {
        System.out.println("run ServiceA...");
    }
}

class ServiceB implements Processor {
    @Override
    public void process() {
        System.out.println("run ServiceB...");
    }
}

public class DynamicRun {

    static class ProcessRunner implements Processor {

        private DelegateProcess target;
        private int runCount;

        private ServiceA defaultProcessor = new ServiceA();

        @Override
        public void process() {
            int count = ++this.runCount;
            System.out.println("默认运行次数：" + count);
            if (count >= 10) {
                System.out.println("默认运行次数到达10次，切换为ServiceB运行");
                // 将真正processor改为ServiceB
                this.target.setDelegate(new ServiceB());
            }

            defaultProcessor.process();
        }

        public void setTarget(DelegateProcess target) {
            this.target = target;
        }

    }

    static class DelegateProcess implements Processor {

        private Processor delegate;

        private DelegateProcess(Processor delegate) {
            this.delegate = delegate;
        }

        @Override
        public void process() {
            delegate.process();
        }

        public void setDelegate(Processor delegate) {
            this.delegate = delegate;
        }

    }

    public static void main(String[] args) {
        // 拿到这个processor，可在其它地方运行，不受约束
        Processor processor = buildProcessor();
        for (int i = 0; i < 15; i++) {
            processor.process();
        }
    }

    private static Processor buildProcessor() {
        ProcessRunner processRunner = new ProcessRunner();
        DelegateProcess delegateProcess = new DelegateProcess(processRunner);
        processRunner.setTarget(delegateProcess);
        return delegateProcess;
    }

}

此方式在JDK中也有出现，可以参考 sun.reflect.ReflectionFactory.newMethodAccessor() 方法。该方法在使用 Method.invoke() 时用到。代码片段如下：

NativeMethodAccessorImpl var2 = new NativeMethodAccessorImpl(var1);
DelegatingMethodAccessorImpl var3 = new DelegatingMethodAccessorImpl(var2);
var2.setParent(var3);