基本结构
ChannelHandler
下面是ChannelHandler的定义:
public interface ChannelHandler {
/**
* Gets called after the {@link ChannelHandler} was added to the actual context and it's ready to handle events.
*/
void handlerAdded(ChannelHandlerContext ctx) throws Exception;
/**
* Gets called after the {@link ChannelHandler} was removed from the actual context and it doesn't handle events
* anymore.
*/
void handlerRemoved(ChannelHandlerContext ctx) throws Exception;
/**
* Gets called if a {@link Throwable} was thrown.
*
* @deprecated is part of {@link ChannelInboundHandler}
*/
@Deprecated
void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception;
/**
* Indicates that the same instance of the annotated {@link ChannelHandler}
* can be added to one or more {@link ChannelPipeline}s multiple times
* without a race condition.
* <p>
* If this annotation is not specified, you have to create a new handler
* instance every time you add it to a pipeline because it has unshared
* state such as member variables.
* <p>
* This annotation is provided for documentation purpose, just like
* <a href="http://www.javaconcurrencyinpractice.com/annotations/doc/">the JCIP annotations</a>.
*/
@Inherited
@Documented
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface Sharable {
// no value
}
}
可见它最关注的事情有三个:
(1).添加Handler
(2).移除Handler
(3).出现异常的时候的处理
@Sharable
对于Sharable注解,上面代码中也有很好的介绍。
如果我们确保我们的实例中不会出现竞态条件,那么就可以使用共享的。
ChannelHandlerAdapter
这个类是对ChannelHandler的基本实现,作为一个适配器,它并没有提供太多的功能,仅仅是处理了是否共享的问题。
下面是其代码:
public abstract class ChannelHandlerAdapter implements ChannelHandler {
// Not using volatile because it's used only for a sanity check.
boolean added;
/**
* Throws {@link IllegalStateException} if {@link ChannelHandlerAdapter#isSharable()} returns {@code true}
*/
protected void ensureNotSharable() {
if (isSharable()) {
throw new IllegalStateException("ChannelHandler " + getClass().getName() + " is not allowed to be shared");
}
}
/**
* Return {@code true} if the implementation is {@link Sharable} and so can be added
* to different {@link ChannelPipeline}s.
*/
public boolean isSharable() {
/**
* Cache the result of {@link Sharable} annotation detection to workaround a condition. We use a
* {@link ThreadLocal} and {@link WeakHashMap} to eliminate the volatile write/reads. Using different
* {@link WeakHashMap} instances per {@link Thread} is good enough for us and the number of
* {@link Thread}s are quite limited anyway.
*
* See <a href="https://github.com/netty/netty/issues/2289">#2289</a>.
*/
Class<?> clazz = getClass();
Map<Class<?>, Boolean> cache = InternalThreadLocalMap.get().handlerSharableCache();
Boolean sharable = cache.get(clazz);
if (sharable == null) {
sharable = clazz.isAnnotationPresent(Sharable.class);
cache.put(clazz, sharable);
}
return sharable;
}
/**
* Do nothing by default, sub-classes may override this method.
*/
@Override
public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
// NOOP
}
/**
* Do nothing by default, sub-classes may override this method.
*/
@Override
public void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
// NOOP
}
/**
* Calls {@link ChannelHandlerContext#fireExceptionCaught(Throwable)} to forward
* to the next {@link ChannelHandler} in the {@link ChannelPipeline}.
*
* Sub-classes may override this method to change behavior.
*/
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
ctx.fireExceptionCaught(cause);
}
}
它对于实现的接口都给了空实现,而且对于共享态的处理也只是提供了基础的工具。