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 given {@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