Android源码--Android设计模式之Abstract Factory

青旅半醒 2023-10-17 10:42 28阅读 0赞

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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