聊一聊Spring中的线程安全性
原文出處：SylvanasSun
- Spring與線程安全
- ThreadLocal
- ThreadLocal中的內(nèi)存泄漏
- 參考文獻(xiàn)
Spring與線程安全
Spring作為一個(gè)IOC/DI容器，幫助我們管理了許許多多的“bean”。但其實(shí)，Spring并沒(méi)有保證這些對(duì)象的線程安全，需要由開(kāi)發(fā)者自己編寫(xiě)解決線程安全問(wèn)題的代碼。
Spring對(duì)每個(gè)bean提供了一個(gè)scope屬性來(lái)表示該bean的作用域。它是bean的生命周期。例如，一個(gè)scope為singleton的bean，在第一次被注入時(shí)，會(huì)創(chuàng)建為一個(gè)單例對(duì)象，該對(duì)象會(huì)一直被復(fù)用到應(yīng)用結(jié)束。
singleton：默認(rèn)的scope，每個(gè)scope為singleton的bean都會(huì)被定義為一個(gè)單例對(duì)象，該對(duì)象的生命周期是與Spring IOC容器一致的（但在第一次被注入時(shí)才會(huì)創(chuàng)建）。
prototype：bean被定義為在每次注入時(shí)都會(huì)創(chuàng)建一個(gè)新的對(duì)象。
request：bean被定義為在每個(gè)HTTP請(qǐng)求中創(chuàng)建一個(gè)單例對(duì)象，也就是說(shuō)在單個(gè)請(qǐng)求中都會(huì)復(fù)用這一個(gè)單例對(duì)象。
session：bean被定義為在一個(gè)session的生命周期內(nèi)創(chuàng)建一個(gè)單例對(duì)象。
application：bean被定義為在ServletContext的生命周期中復(fù)用一個(gè)單例對(duì)象。
websocket：bean被定義為在websocket的生命周期中復(fù)用一個(gè)單例對(duì)象。
我們交由Spring管理的大多數(shù)對(duì)象其實(shí)都是一些無(wú)狀態(tài)的對(duì)象，這種不會(huì)因?yàn)槎嗑€程而導(dǎo)致?tīng)顟B(tài)被破壞的對(duì)象很適合Spring的默認(rèn)scope，每個(gè)單例的無(wú)狀態(tài)對(duì)象都是線程安全的（也可以說(shuō)只要是無(wú)狀態(tài)的對(duì)象，不管單例多例都是線程安全的，不過(guò)單例畢竟節(jié)省了不斷創(chuàng)建對(duì)象與GC的開(kāi)銷(xiāo)）。
無(wú)狀態(tài)的對(duì)象即是自身沒(méi)有狀態(tài)的對(duì)象，自然也就不會(huì)因?yàn)槎鄠€(gè)線程的交替調(diào)度而破壞自身狀態(tài)導(dǎo)致線程安全問(wèn)題。無(wú)狀態(tài)對(duì)象包括我們經(jīng)常使用的DO、DTO、VO這些只作為數(shù)據(jù)的實(shí)體模型的貧血對(duì)象，還有Service、DAO和Controller，這些對(duì)象并沒(méi)有自己的狀態(tài)，它們只是用來(lái)執(zhí)行某些操作的。例如，每個(gè)DAO提供的函數(shù)都只是對(duì)數(shù)據(jù)庫(kù)的CRUD，而且每個(gè)數(shù)據(jù)庫(kù)Connection都作為函數(shù)的局部變量（局部變量是在用戶棧中的，而且用戶棧本身就是線程私有的內(nèi)存區(qū)域，所以不存在線程安全問(wèn)題），用完即關(guān)（或交還給連接池）。
有人可能會(huì)認(rèn)為，我使用request作用域不就可以避免每個(gè)請(qǐng)求之間的安全問(wèn)題了嗎？這是完全錯(cuò)誤的，因?yàn)镃ontroller默認(rèn)是單例的，一個(gè)HTTP請(qǐng)求是會(huì)被多個(gè)線程執(zhí)行的，這就又回到了線程的安全問(wèn)題。當(dāng)然，你也可以把Controller的scope改成prototype，實(shí)際上Struts2就是這么做的，但有一點(diǎn)要注意，Spring MVC對(duì)請(qǐng)求的攔截粒度是基于每個(gè)方法的，而Struts2是基于每個(gè)類(lèi)的，所以把Controller設(shè)為多例將會(huì)頻繁的創(chuàng)建與回收對(duì)象，嚴(yán)重影響到了性能。
通過(guò)閱讀上文其實(shí)已經(jīng)說(shuō)的很清楚了，Spring根本就沒(méi)有對(duì)bean的多線程安全問(wèn)題做出任何保證與措施。對(duì)于每個(gè)bean的線程安全問(wèn)題，根本原因是每個(gè)bean自身的設(shè)計(jì)。不要在bean中聲明任何有狀態(tài)的實(shí)例變量或類(lèi)變量，如果必須如此，那么就使用ThreadLocal把變量變?yōu)榫€程私有的，如果bean的實(shí)例變量或類(lèi)變量需要在多個(gè)線程之間共享，那么就只能使用synchronized、lock、CAS等這些實(shí)現(xiàn)線程同步的方法了。
下面將通過(guò)解析ThreadLocal的源碼來(lái)了解它的實(shí)現(xiàn)與作用，ThreadLocal是一個(gè)很好用的工具類(lèi)，它在某些情況下解決了線程安全問(wèn)題（在變量不需要被多個(gè)線程共享時(shí)）。
ThreadLocal
ThreadLocal是一個(gè)為線程提供線程局部變量的工具類(lèi)。它的思想也十分簡(jiǎn)單，就是為線程提供一個(gè)線程私有的變量副本，這樣多個(gè)線程都可以隨意更改自己線程局部的變量，不會(huì)影響到其他線程。不過(guò)需要注意的是，ThreadLocal提供的只是一個(gè)淺拷貝，如果變量是一個(gè)引用類(lèi)型，那么就要考慮它內(nèi)部的狀態(tài)是否會(huì)被改變，想要解決這個(gè)問(wèn)題可以通過(guò)重寫(xiě)ThreadLocal的initialValue()函數(shù)來(lái)自己實(shí)現(xiàn)深拷貝，建議在使用ThreadLocal時(shí)一開(kāi)始就重寫(xiě)該函數(shù)。
ThreadLocal與像synchronized這樣的鎖機(jī)制是不同的。首先，它們的應(yīng)用場(chǎng)景與實(shí)現(xiàn)思路就不一樣，鎖更強(qiáng)調(diào)的是如何同步多個(gè)線程去正確地共享一個(gè)變量，ThreadLocal則是為了解決同一個(gè)變量如何不被多個(gè)線程共享。從性能開(kāi)銷(xiāo)的角度上來(lái)講，如果鎖機(jī)制是用時(shí)間換空間的話，那么ThreadLocal就是用空間換時(shí)間。
ThreadLocal中含有一個(gè)叫做ThreadLocalMap的內(nèi)部類(lèi)，該類(lèi)為一個(gè)采用線性探測(cè)法實(shí)現(xiàn)的HashMap。它的key為T(mén)hreadLocal對(duì)象而且還使用了WeakReference，ThreadLocalMap正是用來(lái)存儲(chǔ)變量副本的。
/*** ThreadLocalMap is a customized hash map suitable only for* maintaining thread local values. No operations are exported* outside of the ThreadLocal class. The class is package private to* allow declaration of fields in class Thread. To help deal with* very large and long-lived usages, the hash table entries use* WeakReferences for keys. However, since reference queues are not* used, stale entries are guaranteed to be removed only when* the table starts running out of space.*/static class ThreadLocalMap {/*** The entries in this hash map extend WeakReference, using* its main ref field as the key (which is always a* ThreadLocal object). Note that null keys (i.e. entry.get()* == null) mean that the key is no longer referenced, so the* entry can be expunged from table. Such entries are referred to* as "stale entries" in the code that follows.*/static class Entry extends WeakReference<ThreadLocal<?>> {/** The value associated with this ThreadLocal. */Object value;Entry(ThreadLocal<?> k, Object v) {super(k);value = v;}}....}
ThreadLocal中只含有三個(gè)成員變量，這三個(gè)變量都是與ThreadLocalMap的hash策略相關(guān)的。
/*** ThreadLocals rely on per-thread linear-probe hash maps attached* to each thread (Thread.threadLocals and* inheritableThreadLocals). The ThreadLocal objects act as keys,* searched via threadLocalHashCode. This is a custom hash code* (useful only within ThreadLocalMaps) that eliminates collisions* in the common case where consecutively constructed ThreadLocals* are used by the same threads, while remaining well-behaved in* less common cases.*/private final int threadLocalHashCode = nextHashCode();/*** The next hash code to be given out. Updated atomically. Starts at* zero.*/private static AtomicInteger nextHashCode =new AtomicInteger();/*** The difference between successively generated hash codes - turns* implicit sequential thread-local IDs into near-optimally spread* multiplicative hash values for power-of-two-sized tables.*/private static final int HASH_INCREMENT = 0x61c88647;/*** Returns the next hash code.*/private static int nextHashCode() {return nextHashCode.getAndAdd(HASH_INCREMENT);}
唯一的實(shí)例變量threadLocalHashCode是用來(lái)進(jìn)行尋址的hashcode，它由函數(shù)nextHashCode()生成，該函數(shù)簡(jiǎn)單地通過(guò)一個(gè)增量HASH_INCREMENT來(lái)生成hashcode。至于為什么這個(gè)增量為0x61c88647，主要是因?yàn)門(mén)hreadLocalMap的初始大小為16，每次擴(kuò)容都會(huì)為原來(lái)的2倍，這樣它的容量永遠(yuǎn)為2的n次方，該增量選為0x61c88647也是為了盡可能均勻地分布，減少碰撞沖突。
/*** The initial capacity -- MUST be a power of two.*/private static final int INITIAL_CAPACITY = 16; /*** Construct a new map initially containing (firstKey, firstValue).* ThreadLocalMaps are constructed lazily, so we only create* one when we have at least one entry to put in it.*/ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {table = new Entry[INITIAL_CAPACITY];int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);table[i] = new Entry(firstKey, firstValue);size = 1;setThreshold(INITIAL_CAPACITY);}
要獲得當(dāng)前線程私有的變量副本需要調(diào)用get()函數(shù)。首先，它會(huì)調(diào)用getMap()函數(shù)去獲得當(dāng)前線程的ThreadLocalMap，這個(gè)函數(shù)需要接收當(dāng)前線程的實(shí)例作為參數(shù)。如果得到的ThreadLocalMap為null，那么就去調(diào)用setInitialValue()函數(shù)來(lái)進(jìn)行初始化，如果不為null，就通過(guò)map來(lái)獲得變量副本并返回。
setInitialValue()函數(shù)會(huì)去先調(diào)用initialValue()函數(shù)來(lái)生成初始值，該函數(shù)默認(rèn)返回null，我們可以通過(guò)重寫(xiě)這個(gè)函數(shù)來(lái)返回我們想要在ThreadLocal中維護(hù)的變量。之后，去調(diào)用getMap()函數(shù)獲得ThreadLocalMap，如果該map已經(jīng)存在，那么就用新獲得value去覆蓋舊值，否則就調(diào)用createMap()函數(shù)來(lái)創(chuàng)建新的map。
/*** Returns the value in the current thread's copy of this* thread-local variable. If the variable has no value for the* current thread, it is first initialized to the value returned* by an invocation of the {@link #initialValue} method.** @return the current thread's value of this thread-local*/public T get() {Thread t = Thread.currentThread();ThreadLocalMap map = getMap(t);if (map != null) {ThreadLocalMap.Entry e = map.getEntry(this);if (e != null) {@SuppressWarnings("unchecked")T result = (T)e.value;return result;}}return setInitialValue();}/*** Variant of set() to establish initialValue. Used instead* of set() in case user has overridden the set() method.** @return the initial value*/private T setInitialValue() {T value = initialValue();Thread t = Thread.currentThread();ThreadLocalMap map = getMap(t);if (map != null)map.set(this, value);elsecreateMap(t, value);return value;}protected T initialValue() {return null;}
ThreadLocal的set()與remove()函數(shù)要比get()的實(shí)現(xiàn)還要簡(jiǎn)單，都只是通過(guò)getMap()來(lái)獲得ThreadLocalMap然后對(duì)其進(jìn)行操作。
/*** Sets the current thread's copy of this thread-local variable* to the specified value. Most subclasses will have no need to* override this method, relying solely on the {@link #initialValue}* method to set the values of thread-locals.** @param value the value to be stored in the current thread's copy of* this thread-local.*/public void set(T value) {Thread t = Thread.currentThread();ThreadLocalMap map = getMap(t);if (map != null)map.set(this, value);elsecreateMap(t, value);}/*** Removes the current thread's value for this thread-local* variable. If this thread-local variable is subsequently* {@linkplain #get read} by the current thread, its value will be* reinitialized by invoking its {@link #initialValue} method,* unless its value is {@linkplain #set set} by the current thread* in the interim. This may result in multiple invocations of the* {@code initialValue} method in the current thread.** @since 1.5*/public void remove() {ThreadLocalMap m = getMap(Thread.currentThread());if (m != null)m.remove(this);}
getMap()函數(shù)與createMap()函數(shù)的實(shí)現(xiàn)也十分簡(jiǎn)單，但是通過(guò)觀察這兩個(gè)函數(shù)可以發(fā)現(xiàn)一個(gè)秘密：ThreadLocalMap是存放在Thread中的。
/*** Get the map associated with a ThreadLocal. Overridden in* InheritableThreadLocal.** @param t the current thread* @return the map*/ThreadLocalMap getMap(Thread t) {return t.threadLocals;}/*** Create the map associated with a ThreadLocal. Overridden in* InheritableThreadLocal.** @param t the current thread* @param firstValue value for the initial entry of the map*/void createMap(Thread t, T firstValue) {t.threadLocals = new ThreadLocalMap(this, firstValue);}// Thread中的源碼/* ThreadLocal values pertaining to this thread. This map is maintained* by the ThreadLocal class. */ThreadLocal.ThreadLocalMap threadLocals = null;/** InheritableThreadLocal values pertaining to this thread. This map is* maintained by the InheritableThreadLocal class.*/ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
仔細(xì)想想其實(shí)就能夠理解這種設(shè)計(jì)的思想。有一種普遍的方法是通過(guò)一個(gè)全局的線程安全的Map來(lái)存儲(chǔ)各個(gè)線程的變量副本，但是這種做法已經(jīng)完全違背了ThreadLocal的本意，設(shè)計(jì)ThreadLocal的初衷就是為了避免多個(gè)線程去并發(fā)訪問(wèn)同一個(gè)對(duì)象，盡管它是線程安全的。而在每個(gè)Thread中存放與它關(guān)聯(lián)的ThreadLocalMap是完全符合ThreadLocal的思想的，當(dāng)想要對(duì)線程局部變量進(jìn)行操作時(shí)，只需要把Thread作為key來(lái)獲得Thread中的ThreadLocalMap即可。這種設(shè)計(jì)相比采用一個(gè)全局Map的方法會(huì)多占用很多內(nèi)存空間，但也因此不需要額外的采取鎖等線程同步方法而節(jié)省了時(shí)間上的消耗。
ThreadLocal中的內(nèi)存泄漏
我們要考慮一種會(huì)發(fā)生內(nèi)存泄漏的情況，如果ThreadLocal被設(shè)置為null后，而且沒(méi)有任何強(qiáng)引用指向它，根據(jù)垃圾回收的可達(dá)性分析算法，ThreadLocal將會(huì)被回收。這樣一來(lái)，ThreadLocalMap中就會(huì)含有key為null的Entry，而且ThreadLocalMap是在Thread中的，只要線程遲遲不結(jié)束，這些無(wú)法訪問(wèn)到的value會(huì)形成內(nèi)存泄漏。為了解決這個(gè)問(wèn)題，ThreadLocalMap中的getEntry()、set()和remove()函數(shù)都會(huì)清理key為null的Entry，以下面的getEntry()函數(shù)的源碼為例。
/*** Get the entry associated with key. This method* itself handles only the fast path: a direct hit of existing* key. It otherwise relays to getEntryAfterMiss. This is* designed to maximize performance for direct hits, in part* by making this method readily inlinable.** @param key the thread local object* @return the entry associated with key, or null if no such*/private Entry getEntry(ThreadLocal<?> key) {int i = key.threadLocalHashCode & (table.length - 1);Entry e = table[i];if (e != null && e.get() == key)return e;elsereturn getEntryAfterMiss(key, i, e);}/*** Version of getEntry method for use when key is not found in* its direct hash slot.** @param key the thread local object* @param i the table index for key's hash code* @param e the entry at table[i]* @return the entry associated with key, or null if no such*/private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {Entry[] tab = table;int len = tab.length;// 清理key為null的Entrywhile (e != null) {ThreadLocal<?> k = e.get();if (k == key)return e;if (k == null)expungeStaleEntry(i);elsei = nextIndex(i, len);e = tab[i];}return null;}
在上文中我們發(fā)現(xiàn)了ThreadLocalMap的key是一個(gè)弱引用，那么為什么使用弱引用呢？使用強(qiáng)引用key與弱引用key的差別如下：
強(qiáng)引用key：ThreadLocal被設(shè)置為null，由于ThreadLocalMap持有ThreadLocal的強(qiáng)引用，如果不手動(dòng)刪除，那么ThreadLocal將不會(huì)回收，產(chǎn)生內(nèi)存泄漏。
弱引用key：ThreadLocal被設(shè)置為null，由于ThreadLocalMap持有ThreadLocal的弱引用，即便不手動(dòng)刪除，ThreadLocal仍會(huì)被回收，ThreadLocalMap在之后調(diào)用set()、getEntry()和remove()函數(shù)時(shí)會(huì)清除所有key為null的Entry。
但要注意的是，ThreadLocalMap僅僅含有這些被動(dòng)措施來(lái)補(bǔ)救內(nèi)存泄漏問(wèn)題。如果你在之后沒(méi)有調(diào)用ThreadLocalMap的set()、getEntry()和remove()函數(shù)的話，那么仍然會(huì)存在內(nèi)存泄漏問(wèn)題。
在使用線程池的情況下，如果不及時(shí)進(jìn)行清理，內(nèi)存泄漏問(wèn)題事小，甚至還會(huì)產(chǎn)生程序邏輯上的問(wèn)題。所以，為了安全地使用ThreadLocal，必須要像每次使用完鎖就解鎖一樣，在每次使用完ThreadLocal后都要調(diào)用remove()來(lái)清理無(wú)用的Entry。
參考文獻(xiàn)
Are Spring objects thread safe? - Stack Overflow
Spring Singleton, Request, Session Beans and Thread Safety | Java Enterprise Ecosystem.
Spring Framework Documentation
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