Netty中ByteBuf采用引用计数的方式进行资源的回收，当ByteBuf中的引用计数值为0时，Netty则不允许应用程序采用任何的方式对这个资源进行访问。它通过实现ReferenceCounted维护引用计数。
package io.netty.util;
/**
* A reference-counted object that requires explicit deallocation.
* 一个需要显式进行资源回收的引用计数的对象
* <p>
* When a new {@link ReferenceCounted} is instantiated, it starts with the reference count of {@code 1}.
* {@link #retain()} increases the reference count, and {@link #release()} decreases the reference count.
* If the reference count is decreased to {@code 0}, the object will be deallocated explicitly, and accessing
* the deallocated object will usually result in an access violation.
* 当一个新的引用计数被实例化时，它的引用计数是从1开始的。通过retain()来增加引用计数，以及release()方法减少引用计数。
* 当这个引用计数减少到0时，这个对象将会被显式的回收，并且访问这个被回收的对象通常会导致一个访问违背。
* </p>
* <p>
* If an object that implements {@link ReferenceCounted} is a container of other objects that implement
* {@link ReferenceCounted}, the contained objects will also be released via {@link #release()} when the container's
* reference count becomes 0.
* 如果一个实现了ReferenceCounted接口的对象是一个其他实现了ReferenceCounted对象的容器，
* 当这个容器的引用计数变成0的话，那么这些被包含的对象也会通过通过调用release()方法被释放掉，
* </p>
*/
public interface ReferenceCounted {
/**
* Returns the reference count of this object. If {@code 0}, it means this object has been deallocated.
* 返回这个对象的引用计数，当为0的时候，它以为着这个对象已经被回收了
*/
int refCnt();
/**
* Increases the reference count by {@code 1}.
* 增加引用计数，增加幅度为1
*/
ReferenceCounted retain();
/**
* Increases the reference count by the specified {@code increment}.
* 增加引用计数，增加幅度为特定的increment参数值。
*/
ReferenceCounted retain(int increment);
/**
* Decreases the reference count by {@code 1} and deallocates this object if the reference count reaches at
* {@code 0}.
* 减少引用计数，减少1，如果引用计数达到0的话，则回收这个对象
* @return {@code true} if and only if the reference count became {@code 0} and this object has been deallocated
*/
boolean release();
/**
* Decreases the reference count by the specified {@code decrement} and deallocates this object if the reference
* count reaches at {@code 0}.
* 减少引用计数，减少指定的decrement值，如果引用计数达到0的话，则回收这个对象
* @return {@code true} if and only if the reference count became {@code 0} and this object has been deallocated
*/
boolean release(int decrement);
}