定义

中介者模式（Mediator），作为中介者使得两个存在耦合关系的对象互不相干，将多对多的相互作用转化为一对多的相互作用。

实现

定义一个中介者抽象类：

public abstract class Mediator {
    public abstract void exec(String command);
}

再定义一个持有中介者的抽象角色类，这里以房东和租客的例子展开：

public abstract class Role {

    protected Mediator mediator;

    public Role(Mediator mediator) {
        this.mediator = mediator;
    }
}

租客类：

public class Renter extends Role {

    public Renter(Mediator mediator) {
        super(mediator);
    }

    public void receive() {
        System.out.println("租户收到新的可租信息");
    }

    public void requestRent() {
        System.out.println("租户请求租房");
    }
}

房东类：

public class Landlord extends Role {

    public Landlord(Mediator mediator) {
        super(mediator);
    }

    public void rentOut() {
        System.out.println("房东发布出租信息");
    }

    public void receive() {
        System.out.println("房东收到新的求租信息");
    }
}

中介掌握了房东和租客的信息，并进行双向同步，其中介者的作用便展现了出来：

public class OneMediator extends Mediator {

    private Landlord landlord;
    private Renter renter;

    public OneMediator() {

    }

    @Override
    public void exec(String command) {
        if (command.equals("landlord")) {
            landlord.rentOut();
            renter.receive();
        } else if (command.equals("renter")) {
            renter.requestRent();
            landlord.receive();
        }
    }

    public Landlord getLandlord() {
        return landlord;
    }

    public void setLandlord(Landlord landlord) {
        this.landlord = landlord;
    }

    public Renter getRenter() {
        return renter;
    }

    public void setRenter(Renter renter) {
        this.renter = renter;
    }
}

public class Client {

    public static void main(String[] args) {
        OneMediator mediator = new OneMediator();

        Landlord landlord = new Landlord(mediator);
        Renter renter = new Renter(mediator);

        mediator.setLandlord(landlord);
        mediator.setRenter(renter);

        mediator.exec("landlord");
        mediator.exec("renter");
    }
}

从例子中可以看出，中介者持有原本具有紧耦合的多对多关系的房东和租客，实现了两者的松耦合。通过中介者解决了这种复杂的网状关系，转而实现了以中介者为中心的星型结构，降低了系统复杂性，提高了可扩展性。

Android 源码中的中介者模式

public class KeyguardViewMediator extends SystemUI {
    private AlarmManager mAlarmManager;
    private AudioManager mAudioManager;
    private StatusBarManager mStatusBarManager;
    private boolean mSwitchingUser;

    private boolean mSystemReady;
    private boolean mBootCompleted;

    /** High level access to the power manager for WakeLocks */
   private PowerManager mPM;

   /** High level access to the window manager for dismissing keyguard animation */
    private IWindowManager mWM;


   /** TrustManager for letting it know when we change visibility */
    private TrustManager mTrustManager;

    /** SearchManager for determining whether or not search assistant is available */
    private SearchManager mSearchManager;
}

本文由 Fynn_ 原创，未经许可，不得转载！