Shiro学习笔记（一）ThreadContext源码解析

最近有空，看了下shiro源码，这里写下来用以加深记忆。本篇记录下ThreadContext类的源码解析。

本篇用到的ThreadLocal及ThreadLocalMap的相关知识戳这里。

首先看下ThreadContext类的说明：

/**
 * A ThreadContext provides a means of binding and unbinding objects to the
 * current thread based on key/value pairs.
 * 

 * 
An internal {@link java.util.HashMap} is used to maintain the key/value pairs
 * for each thread.
 * 

 * 
If the desired behavior is to ensure that bound data is not shared across
 * threads in a pooled or reusable threaded environment, the application (or more likely a framework) must
 * bind and remove any necessary values at the beginning and end of stack
 * execution, respectively (i.e. individually explicitly or all via the clear method).
 *
 * @see #remove()
 * @since 0.1
 */
public abstract class ThreadContext {

ThreadContext类通过key/value键值对为每一个线程提供绑定及解绑对象的方法。如果使用了如线程池这样的技术并且不希望共享数据，那么在使用ThreadContext的开始和结束阶段都需清除数据。

    /**
     * Private internal log instance.
     */
    private static final Logger log = LoggerFactory.getLogger(ThreadContext.class);

    public static final String SECURITY_MANAGER_KEY = ThreadContext.class.getName() + "_SECURITY_MANAGER_KEY";
    public static final String SUBJECT_KEY = ThreadContext.class.getName() + "_SUBJECT_KEY";

    private static final ThreadLocal> resources = new InheritableThreadLocalMap>();

ThreadContext的静态变量共有四个，第一个用于记录日志这个没什么好说的，第二个SECURITY_MANAGER_KEY是用来表示SecurityManager对象对应的key，SUBJECT_KEY则是表示Subject对象对应的key，这里是通过反射来定义这两个key的值。最后一个resources对象实际上就是一个ThreadLocal。通过这四行代码我们大致可以了解ThreadContext类的作用。ThreadContext的作用其实就是通过每个线程的ThreadLocalMap来存储线程自己的SecurityManager对象以及Subject对象，因为用到了ThreadLocal因此这里的操作都是线程安全地。

getResources方法

    /**
     * Returns the ThreadLocal Map. This Map is used internally to bind objects
     * to the current thread by storing each object under a unique key.
     *
     * @return the map of bound resources
     */
    public static Map getResources() {
        if (resources.get() == null){
            return Collections.emptyMap();
        } else {
            return new HashMap(resources.get());
        }
    }

getResources方法用于返回当前线程存储在ThreadLocalMap中的HashMap对象，正常的话这个hashmap对象中应该包含有Security以及Subject对象的键值对。如果返回的hashmap为空则返回一个空的hashmap对象。

setResources方法

    /**
     * Allows a caller to explicitly set the entire resource map.  This operation overwrites everything that existed
     * previously in the ThreadContext - if you need to retain what was on the thread prior to calling this method,
     * call the {@link #getResources()} method, which will give you the existing state.
     *
     * @param newResources the resources to replace the existing {@link #getResources() resources}.
     * @since 1.0
     */
    public static void setResources(Map newResources) {
        if (CollectionUtils.isEmpty(newResources)) {
            return;
        }
        ensureResourcesInitialized();
        resources.get().clear();
        resources.get().putAll(newResources);
    }

    private static void ensureResourcesInitialized(){
        if (resources.get() == null){
           resources.set(new HashMap());
        }
    }

setResources可用于设置新的map对象至resources对象也就是ThreadLocal对象中。这里首先保证新的map对象不为空，之后保证resources里的value也就是hashmap对象已经存在，然后为防止之前的数据影响，先调用clear方法清除数据然后再调用hashmap的putAll方法将新的map存入ThreadLocal对应的hashmap中。

getValue方法

    /**
     * Returns the value bound in the {@code ThreadContext} under the specified {@code key}, or {@code null} if there
     * is no value for that {@code key}.
     *
     * @param key the map key to use to lookup the value
     * @return the value bound in the {@code ThreadContext} under the specified {@code key}, or {@code null} if there
     *         is no value for that {@code key}.
     * @since 1.0
     */
    private static Object getValue(Object key) {
        Map perThreadResources = resources.get();
        return perThreadResources != null ? perThreadResources.get(key) : null;
    }

getValue方法很简单，首先通过ThreadLocal方法的get方法获取到hashmap对象然后调用hashmap对象获取对应的value。

get方法

    /**
     * Returns the object for the specified key that is bound to
     * the current thread.
     *
     * @param key the key that identifies the value to return
     * @return the object keyed by key or null if
     *         no value exists for the specified key
     */
    public static Object get(Object key) {
        if (log.isTraceEnabled()) {
            String msg = "get() - in thread [" + Thread.currentThread().getName() + "]";
            log.trace(msg);
        }

        Object value = getValue(key);
        if ((value != null) && log.isTraceEnabled()) {
            String msg = "Retrieved value of type [" + value.getClass().getName() + "] for key [" +
                    key + "] " + "bound to thread [" + Thread.currentThread().getName() + "]";
            log.trace(msg);
        }
        return value;
    }

getValue方法是ThreadContext私有方法而get方法则是ThreadContext类对外提供的获取值的方法，这里也什么好说的只是单纯的记录了下调用getValue方法的log。

put方法

    /**
     * Binds value for the given key to the current thread.
     * 

     * 
A null value has the same effect as if remove was called for the given
     * key, i.e.:
     * 

     * 
     * if ( value == null ) {
     *     remove( key );
     * }
     *
     * @param key   The key with which to identify the value.
     * @param value The value to bind to the thread.
     * @throws IllegalArgumentException if the key argument is null.
     */
    public static void put(Object key, Object value) {
        if (key == null) {
            throw new IllegalArgumentException("key cannot be null");
        }

        if (value == null) {
            remove(key);
            return;
        }

        ensureResourcesInitialized();
        resources.get().put(key, value);

        if (log.isTraceEnabled()) {
            String msg = "Bound value of type [" + value.getClass().getName() + "] for key [" +
                    key + "] to thread " + "[" + Thread.currentThread().getName() + "]";
            log.trace(msg);
        }
    }

put方法首先判断key是不是null，如果是则抛出异常。如果value是null，这里算是一个能够在正常环境中使用的小技巧，当我们想删除key对应的键值对时，可以调用put方法并将value传入null。两个基本的键值判断之后就是正常的调用hashmap的put方法获取对应的value。

remove方法

    /**
     * Unbinds the value for the given key from the current
     * thread.
     *
     * @param key The key identifying the value bound to the current thread.
     * @return the object unbound or null if there was nothing bound
     *         under the specified key name.
     */
    public static Object remove(Object key) {
        Map perThreadResources = resources.get();
        Object value = perThreadResources != null ? perThreadResources.remove(key) : null;

        if ((value != null) && log.isTraceEnabled()) {
            String msg = "Removed value of type [" + value.getClass().getName() + "] for key [" +
                    key + "]" + "from thread [" + Thread.currentThread().getName() + "]";
            log.trace(msg);
        }

        return value;
    }

remove方法用于解绑key也就是删除key。注意这里remove方法会返回被删除的键值对。

操作SecurityManager及Subject的方法

    /**
     * Convenience method that simplifies retrieval of the application's SecurityManager instance from the current
     * thread. If there is no SecurityManager bound to the thread (probably because framework code did not bind it
     * to the thread), this method returns null.
     * 

     * It is merely a convenient wrapper for the following:
     * 

     * return (SecurityManager)get( SECURITY_MANAGER_KEY );
     * 

     * This method only returns the bound value if it exists - it does not remove it
     * from the thread.  To remove it, one must call {@link #unbindSecurityManager() unbindSecurityManager()} instead.
     *
     * @return the Subject object bound to the thread, or null if there isn't one bound.
     * @since 0.9
     */
    public static SecurityManager getSecurityManager() {
        return (SecurityManager) get(SECURITY_MANAGER_KEY);
    }


    /**
     * Convenience method that simplifies binding the application's SecurityManager instance to the ThreadContext.
     * 

     * 
The method's existence is to help reduce casting in code and to simplify remembering of
     * ThreadContext key names.  The implementation is simple in that, if the SecurityManager is not null,
     * it binds it to the thread, i.e.:
     * 

     * 
     * if (securityManager != null) {
     *     put( SECURITY_MANAGER_KEY, securityManager);
     * }
     *
     * @param securityManager the application's SecurityManager instance to bind to the thread.  If the argument is
     *                        null, nothing will be done.
     * @since 0.9
     */
    public static void bind(SecurityManager securityManager) {
        if (securityManager != null) {
            put(SECURITY_MANAGER_KEY, securityManager);
        }
    }

    /**
     * Convenience method that simplifies removal of the application's SecurityManager instance from the thread.
     * 

     * The implementation just helps reduce casting and remembering of the ThreadContext key name, i.e it is
     * merely a convenient wrapper for the following:
     * 

     * return (SecurityManager)remove( SECURITY_MANAGER_KEY );
     * 

     * If you wish to just retrieve the object from the thread without removing it (so it can be retrieved later
     * during thread execution), use the {@link #getSecurityManager() getSecurityManager()} method instead.
     *
     * @return the application's SecurityManager instance previously bound to the thread, or null if there
     *         was none bound.
     * @since 0.9
     */
    public static SecurityManager unbindSecurityManager() {
        return (SecurityManager) remove(SECURITY_MANAGER_KEY);
    }

    /**
     * Convenience method that simplifies retrieval of a thread-bound Subject.  If there is no
     * Subject bound to the thread, this method returns null.  It is merely a convenient wrapper
     * for the following:
     * 

     * return (Subject)get( SUBJECT_KEY );
     * 

     * This method only returns the bound value if it exists - it does not remove it
     * from the thread.  To remove it, one must call {@link #unbindSubject() unbindSubject()} instead.
     *
     * @return the Subject object bound to the thread, or null if there isn't one bound.
     * @since 0.2
     */
    public static Subject getSubject() {
        return (Subject) get(SUBJECT_KEY);
    }


    /**
     * Convenience method that simplifies binding a Subject to the ThreadContext.
     * 

     * 
The method's existence is to help reduce casting in your own code and to simplify remembering of
     * ThreadContext key names.  The implementation is simple in that, if the Subject is not null,
     * it binds it to the thread, i.e.:
     * 

     * 
     * if (subject != null) {
     *     put( SUBJECT_KEY, subject );
     * }
     *
     * @param subject the Subject object to bind to the thread.  If the argument is null, nothing will be done.
     * @since 0.2
     */
    public static void bind(Subject subject) {
        if (subject != null) {
            put(SUBJECT_KEY, subject);
        }
    }

    /**
     * Convenience method that simplifies removal of a thread-local Subject from the thread.
     * 

     * The implementation just helps reduce casting and remembering of the ThreadContext key name, i.e it is
     * merely a convenient wrapper for the following:
     * 

     * return (Subject)remove( SUBJECT_KEY );
     * 

     * If you wish to just retrieve the object from the thread without removing it (so it can be retrieved later during
     * thread execution), you should use the {@link #getSubject() getSubject()} method for that purpose.
     *
     * @return the Subject object previously bound to the thread, or null if there was none bound.
     * @since 0.2
     */
    public static Subject unbindSubject() {
        return (Subject) remove(SUBJECT_KEY);
    }

这里6个方法就是ThreadContext所提供的6个核心方法，getXXX方法用于返回指定的对象，bind方法用于将对应的值即SecurityManager对象及Subject对象存入线程的ThreadLocalMap对象中，unbind方法则用于清除SecurityManager对象及Subject对象。

最后总结一张ThreadContext类和线程的关系。