Servlet是单例还是多例

这篇文章主要介绍“Servlet是单例还是多例”，在日常操作中，相信很多人在Servlet是单例还是多例问题上存在疑惑，小编查阅了各式资料，整理出简单好用的操作方法，希望对大家解答”Servlet是单例还是多例”的疑惑有所帮助！接下来，请跟着小编一起来学习吧！

在Servlet规范中，对于Servlet单例与多例定义如下：

“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.

上面规范提到，

• 如果一个Servlet没有被部署在分布式的环境中，一般web.xml中声明的一个Servlet只对应一个实例。

  
  

• 而如果一个Servlet实现了SingleThreadModel接口，就会被初始化多个实例。实例有多少呢，这里没细说。

下面再从Tomcat的源码中找寻下具体的参考实现是什么样子的。以下代码来源于Tomcat的StandardWrapper类。我把其中不太相关的部分做了删除。

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); //如果实现STM接口，就放到一个栈里

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; //注意此处，如果存在实例就直接返回

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<>();

} //此处，使用Stack存放STM的Servlet

singleThreadModel = true;

}

initServlet(servlet);

} finally {

}

return servlet;

}

那一个实现了SingleThreadModel接口的Servlet，一般会初始化多少个实例呢？

StandardWrapper类中有两个属性，其中maxInstance初始为20。所以上面的问题就有了答案。

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

/**

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

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