I/O Filter Chain层是介于I/O Service层与I/O Handler层之间的一层，从它的命名上可以看出，这个层可以根据实际应用的需要，设置一组IoFilter来对I/O Service层与I/O Handler层之间传输数据进行过滤，任何需要在这两层之间进行处理的逻辑都可以放到IoFilter中。
我们看一下IoFilter的抽象层次设计，如图所示：
我们看一下IoFilter的抽象层次设计，如图所示：
通过上述类图可见，要实现一个自定义的IoFilter，一般是直接实现IoFilterAdapter类。同时，Mina也给出了几类常用的开发IoFilter的实现类，如下所示：
- LoggingFilter记录所有事件和请求
- ProtocolCodecFilter将到来的ByteBuffer转换成消息对象（POJO）
- CompressionFilter压缩数据
- SSLFilter增加SSL – TLS – StartTLS支持
想要实现一个自定义的IoFilter实现类，只需要基于上述给出的几个实现类即可。
如果想要实现自己的IoFilter，可以参考如下例子：
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public class MyFilter extends IoFilterAdapter { |
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@Override |
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public void sessionOpened(NextFilter nextFilter, IoSession session) throwsException { |
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// Some logic here... |
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nextFilter.sessionOpened(session); |
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// Some other logic here... |
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} |
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} |
下面通过一个例子来说明，如何使用IoFilter的实现类。
ProtocolCodecFilter
下面是Mina自带的例子，使用了ProtocolCodecFilter类：
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package org.apache.mina.example.gettingstarted.timeserver; |
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import java.io.IOException; |
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import java.net.InetSocketAddress; |
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import java.nio.charset.Charset; |
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import org.apache.mina.core.service.IoAcceptor; |
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import org.apache.mina.core.session.IdleStatus; |
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import org.apache.mina.filter.codec.ProtocolCodecFilter; |
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import org.apache.mina.filter.codec.textline.TextLineCodecFactory; |
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import org.apache.mina.filter.logging.LoggingFilter; |
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import org.apache.mina.transport.socket.nio.NioSocketAcceptor; |
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public class MinaTimeServer { |
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/** |
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* We will use a port above 1024 to be able to launch the server with a |
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* standard user |
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*/ |
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private static final int PORT = 9123; |
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/** |
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* The server implementation. It's based on TCP, and uses a logging filter |
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* plus a text line decoder. |
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*/ |
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public static void main(String[] args) throws IOException { |
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// Create the acceptor |
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IoAcceptor acceptor = new NioSocketAcceptor(); |
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// Add two filters : a logger and a codec |
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acceptor.getFilterChain().addLast("logger", new LoggingFilter()); |
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acceptor.getFilterChain().addLast("codec", new ProtocolCodecFilter(newTextLineCodecFactory(Charset.forName("UTF-8")))); |
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// Attach the business logic to the server |
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acceptor.setHandler(new TimeServerHandler()); |
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// Configurate the buffer size and the iddle time |
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acceptor.getSessionConfig().setReadBufferSize(2048); |
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acceptor.getSessionConfig().setIdleTime(IdleStatus.BOTH_IDLE, 10); |
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// And bind ! |
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acceptor.bind(new InetSocketAddress(PORT)); |
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} |
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} |
上面设置了两个IoFilter，关键是看如果基于文本行的消息，使用一个ProtocolCodecFilter包裹了一TextLineCodecFactory类的实例，使用起来非常容易。
构造一个ProtocolCodecFilter实例，需要实现一个ProtocolCodecFactory实例，一个ProtocolCodecFactory包含了对消息进行编解码（Codec）的逻辑，这样实现的好处是将编解码的逻辑和IoFilter解耦合。下面看一下类图：
LoggingFilter
如果需要记录通信过程中的事件以及请求，则可以直接使用LoggingFilter类，使用方法可以参考上面的例子。
CompressionFilter
CompressionFilter是与压缩/解压缩数据相关的IoFilter，我们可以看一下该类的构造方法，如下所示：
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/** |
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* Creates a new instance which compresses outboud data and decompresses |
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* inbound data with default compression level. |
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*/ |
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public CompressionFilter() { |
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this(true, true, COMPRESSION_DEFAULT); |
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} |
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/** |
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* Creates a new instance which compresses outboud data and decompresses |
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* inbound data with the specified <tt>compressionLevel</tt>. |
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* |
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* @param compressionLevel the level of compression to be used. Must |
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*/ |
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public CompressionFilter(final int compressionLevel) { |
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this(true, true, compressionLevel); |
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} |
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/** |
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* Creates a new instance. |
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* |
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* @param compressInbound <tt>true</tt> if data read is to be decompressed |
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* @param compressOutbound <tt>true</tt> if data written is to be compressed |
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* @param compressionLevel the level of compression to be used. Must |
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*/ |
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public CompressionFilter(final boolean compressInbound, final booleancompressOutbound, final int compressionLevel) { |
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this.compressionLevel = compressionLevel; |
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this.compressInbound = compressInbound; |
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this.compressOutbound = compressOutbound; |
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} |
基本上就构造方法参数中指定的3个参数与压缩/解压缩相关：
- compressionLevel
- compressInbound
- compressOutbound
使用的时候也比较简单，只需要创建一个CompressionFilter实例，加入到Filter Chain中即可。
DefaultIoFilterChainBuilder
Mina自带的DefaultIoFilterChainBuilder可以非常容易就可以构建一个Filter Chain，默认在创建IoAcceptor和IoConnector的时候，可以直接通过他们获取到一个DefaultIoFilterChainBuilder的实例，然后调用add*方法设置IoFilter链，如下面代码中示例：
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IoAcceptor acceptor = new NioSocketAcceptor(); |
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// Add two filters : a logger and a codec |
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acceptor.getFilterChain().addLast("logger", new LoggingFilter()); |
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acceptor.getFilterChain().addLast("codec", new ProtocolCodecFilter(newTextLineCodecFactory(Charset.forName("UTF-8")))); |
下面看一下来自Mina官网的表格，Mina框架也给出了一些典型的IoFilter的实现，引用如下所示：
| Filter | class | Description |
|---|---|---|
| Blacklist | BlacklistFilter | Blocks connections from blacklisted remote addresses |
| Buffered Write | BufferedWriteFilter | Buffers outgoing requests like the BufferedOutputStream does |
| Compression | CompressionFilter | |
| ConnectionThrottle | ConnectionThrottleFilter | |
| ErrorGenerating | ErrorGeneratingFilter | |
| Executor | ExecutorFilter | |
| FileRegionWrite | FileRegionWriteFilter | |
| KeepAlive | KeepAliveFilter | |
| Logging | LoggingFilter | Logs event messages, like MessageReceived, MessageSent, SessionOpened, … |
| MDC Injection | MdcInjectionFilter | Inject key IoSession properties into the MDC |
| Noop | NoopFilter | A filter that does nothing. Useful for tests. |
| Profiler | ProfilerTimerFilter | Profile event messages, like MessageReceived, MessageSent, SessionOpened, … |
| ProtocolCodec | ProtocolCodecFilter | A filter in charge of encoding and decoding messages |
| Proxy | ProxyFilter | |
| Reference counting | ReferenceCountingFilter | Keeps track of the number of usages of this filter |
| RequestResponse | RequestResponseFilter | |
| SessionAttributeInitializing | SessionAttributeInitializingFilter | |
| StreamWrite | StreamWriteFilter | |
| SslFilter | SslFilter | |
| WriteRequest | WriteRequestFilter |
