Servlet是单例还是多例
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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 SingleThreadModelinterface, 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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