Servlet是單例還是多例

這篇文章主要介紹“Servlet是單例還是多例”，在日常操作中，相信很多人在Servlet是單例還是多例問題上存在疑惑，小編查閱了各式資料，整理出簡單好用的操作方法，希望對大家解答”Servlet是單例還是多例”的疑惑有所幫助！接下來，請跟著小編一起來學習吧！

在Servlet規范中，對于Servlet單例與多例定義如下：

“Deployment Descriptor”, controls how the servlet container provides instances of the servlet.For a servlet not hosted in a distributed environment (the default), the servlet container must use only one instance per servlet declaration. However, for a servlet implementing the SingleThreadModel interface, the servlet container may instantiate multiple instances to handle a heavy request load and serialize requests to a particular instance.

上面規范提到，

• 如果一個Servlet沒有被部署在分布式的環境中，一般web.xml中聲明的一個Servlet只對應一個實例。

  
  

• 而如果一個Servlet實現了SingleThreadModel接口，就會被初始化多個實例。實例有多少呢，這里沒細說。

下面再從Tomcat的源碼中找尋下具體的參考實現是什么樣子的。以下代碼來源于Tomcat的StandardWrapper類。我把其中不太相關的部分做了刪除。

public Servlet allocate() throws ServletException {

boolean newInstance = false;

if (!singleThreadModel) {

// Load and initialize our instance if necessary

if (instance == null) {

synchronized (this) {

if (instance == null) {

try {

instance = loadServlet();

} catch (ServletException e) {}}}}

if (singleThreadModel) {

if (newInstance) {

synchronized (instancePool) {

instancePool.push(instance); //如果實現STM接口，就放到一個棧里

nInstances++;

}}

} else {

if (!newInstance) {

countAllocated.incrementAndGet();

}

return (instance);

}

}

synchronized (instancePool) {

while (countAllocated.get() >= nInstances) {

// Allocate a new instance if possible, or else wait

if (nInstances < maxInstances) {

try {

instancePool.push(loadServlet());

nInstances++;

} catch (ServletException e) {}

} else {

try {

instancePool.wait();

} catch (InterruptedException e) {

// Ignore

}} }

countAllocated.incrementAndGet();

return instancePool.pop();

}}

/**

* Load and initialize an instance of this servlet, if there is not already

* at least one initialized instance. This can be used, for example, to

* load servlets that are marked in the deployment descriptor to be loaded

* at server startup time.

*/

public synchronized Servlet loadServlet() throws ServletException {

// Nothing to do if we already have an instance or an instance pool

if (!singleThreadModel && (instance != null))

return instance; //注意此處，如果存在實例就直接返回

Servlet servlet;

try {

InstanceManager instanceManager = ((StandardContext)getParent()).getInstanceManager();

try {

servlet = (Servlet) instanceManager.newInstance(servletClass);

} catch (ClassCastException e) {

}

if (servlet instanceof SingleThreadModel) {

if (instancePool == null) {

instancePool = new Stack<>();

} //此處，使用Stack存放STM的Servlet

singleThreadModel = true;

}

initServlet(servlet);

} finally {

}

return servlet;

}

那一個實現了SingleThreadModel接口的Servlet，一般會初始化多少個實例呢？

StandardWrapper類中有兩個屬性，其中maxInstance初始為20。所以上面的問題就有了答案。

/**
 * Does this servlet implement the SingleThreadModel interface?
 */
protected volatile boolean singleThreadModel = false;

/**

 * Maximum number of STM instances.
 */
protected int maxInstances = 20;

到此，關于“Servlet是單例還是多例”的學習就結束了，希望能夠解決大家的疑惑。理論與實踐的搭配能更好的幫助大家學習，快去試試吧！若想繼續學習更多相關知識，請繼續關注億速云網站，小編會繼續努力為大家帶來更多實用的文章！