Android源码--Android设计模式之Abstract Factory

本节举例说明Abstract Factory在Android源码中的体现。
abstract factory是对象的创建型模式。这种设计模式在Android中运用相当广泛。现仅仅以Android平台JVM的DatagramSocket的实现为例来介绍之。

DatagramSocket是一个实现了UDP socket的类。

相关代码：

//接口的定义
  1）. SocketOptions.java:
  public interface SocketOptions { ...}
 

//抽象类的定义，必须实现接口SocketOptions
  2）. DatagramSocketImpl.java:
  public abstract class DatagramSocketImpl implements SocketOptions { ...}
 

//抽象类的定义，必须实现接口SocketOptions
  3）. SocketImpl.java:
  public abstract class SocketImpl implements SocketOptions { ...}
 

//抽象工厂接口的定义
  4. DatagramSocketImplFactory.java:
  public interface DatagramSocketImplFactory {
      DatagramSocketImpl createDatagramSocketImpl();
  }


 //抽象工厂接口的定义
  5）. SocketImplFactory.java
  public interface SocketImplFactory {
    SocketImpl createSocketImpl();
  }
 //DatagramSocket类的定义
  6）. DatagramSocket.java


public class DatagramSocket implements Closeable {

    DatagramSocketImpl impl;

    InetAddress address;

    int port = -1;

    static DatagramSocketImplFactory factory;

......

    //创建socket
    synchronized void createSocket(int aPort, InetAddress addr) throws SocketException {
        impl = factory != null ? factory.createDatagramSocketImpl()
                : new PlainDatagramSocketImpl();
        impl.create();
        try {
            impl.bind(aPort, addr);
            isBound = true;
        } catch (SocketException e) {
            close();
            throw e;
        }
    }

    //构造函数
	public DatagramSocket(int aPort) throws SocketException {
        checkPort(aPort);
        createSocket(aPort, Inet4Address.ANY);
    }

    ////构造函数
	public DatagramSocket(int aPort, InetAddress addr) throws SocketException {
        checkPort(aPort);
        createSocket(aPort, (addr == null) ? Inet4Address.ANY : addr);
    }

......

}

简要说明：

从各个类名字的命名就可以看出来，这是典型的abstract factory模式。这里，DatagramSocket就是client。

应用程序通过new DatagramSocket来创建一个Socket。 在DatagramSocket的构造函数中，依次调用

createSocket ,而在这个函数中，会new PlainDatagramSocketImpl。

synchronized void createSocket(int aPort, InetAddress addr) throws SocketException {
        impl = factory != null ? factory.createDatagramSocketImpl()
                : new PlainDatagramSocketImpl();
        impl.create();
        try {
            impl.bind(aPort, addr);
            isBound = true;
        } catch (SocketException e) {
            close();
            throw e;
        }
    }

PlainDatagramSocketImpl的源码如下：

public class PlainDatagramSocketImpl extends DatagramSocketImpl {

......

    //构造函数
    public PlainDatagramSocketImpl(FileDescriptor fd, int localPort) {
        this.fd = fd;
        this.localPort = localPort;
        if (fd.valid()) {
            guard.open("close");
        }
    }

    //构造函数
    public PlainDatagramSocketImpl() {
        fd = new FileDescriptor();
    }

    //绑定
    @Override public void bind(int port, InetAddress address) throws SocketException {
        ......
    }

    @Override
    protected void onBind(InetAddress localAddress, int localPort) {
        this.localPort = localPort;
    }

    //关闭Socket
    @Override
    public synchronized void close() {
        guard.close();
        try {
            IoBridge.closeAndSignalBlockedThreads(fd);
        } catch (IOException ignored) {
        }
    }

    @Override
    protected void onClose() {
        guard.close();
    }

    //创建
    @Override
    public void create() throws SocketException {
        this.fd = IoBridge.socket(false);
    }
    
    //接收
    @Override
    public void receive(DatagramPacket pack) throws IOException {
        doRecv(pack, 0);
    }

    //发送
    @Override
    public void send(DatagramPacket packet) throws IOException {
        int port = isNativeConnected ? 0 : packet.getPort();
        InetAddress address = isNativeConnected ? null : packet.getAddress();
        IoBridge.sendto(fd, packet.getData(), packet.getOffset(), packet.getLength(), 0, address,
            port);
    }

    //连接
    @Override
    public void connect(InetAddress inetAddr, int port) throws SocketException {
        ......
    }

    @Override
    protected void onConnect(InetAddress remoteAddress, int remotePort) {
        isNativeConnected = true;
        connectedAddress = remoteAddress;
        connectedPort = remotePort;
    }

    //断开
    @Override
    public void disconnect() {
        ......
    }
......
}

PlainDatagramSocketImpl是DatagramSocketImpl的具体类，一个典型的client。

 

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