求问这个泛型是啥意思？

最近在学习 netty，但是某些泛型的高级用法没人教的话感觉看不懂。比如这样的

package io.netty.util; /** * Holds {@link Attribute}s which can be accessed via {@link AttributeKey}. * * Implementations must be Thread-safe. */ public interface AttributeMap { /** * Get the {@link Attribute} for the gven {@link AttributeKey}. This method will never return null, but may return * an {@link Attribute} which does not have a value set yet. */ <T> Attribute<T> attr(AttributeKey<T> key); /** * Returns {@code} true if and only if the given {@link Attribute} exists in this {@link AttributeMap}. */ <T> boolean hasAttr(AttributeKey<T> key); } 

在 boolen 之前的<T>是啥意思，有达人知道么？

求教