jakarta-ee - CDI 注入(inject)在 Servlet 中不起作用

标签 jakarta-ee servlets jetty cdi jboss-weld

我真的需要你的帮助。我现在正在处理这个问题数周或数月。我为这篇长文章道歉，但我想尽可能准确地解释问题和我的设置。

我的 JAVA EE 6 Web 应用程序主要包含 3 个 Servlet:一个 javax.faces.webapp.FacesServlet ，一个 StreamingServlet 扩展 javax.servlet.http.HttpServlet 和一个 ClientServlet 扩展 org.eclipse.jetty.websocket.WebSocketServlet .我想将 CDI 与这些 Servlet 一起使用，但它仅适用于 FacesServlet，更准确地说，适用于 JSF 操作路由到的 Bean。 CDI 注入(inject)不适用于其他 2 个 Servlet。

我的项目设置如下:Maven 3.0.4、Jetty 8.1.4(jetty-maven-plugin)、MyFaces 2.1.6、Weld 1.1.8。

文件 pom.xml 包含 CDI 的依赖项:

...
<dependency>
    <groupId>javax.enterprise</groupId>
    <artifactId>cdi-api</artifactId>
    <version>1.1.EDR1.2</version>
    <scope>provided</scope>
</dependency>
<dependency>
    <groupId>javax.annotation</groupId>
    <artifactId>jsr250-api</artifactId>
    <version>1.0</version>
</dependency>
<dependency>
    <groupId>org.jboss.weld.servlet</groupId>
    <artifactId>weld-servlet</artifactId>
    <version>${weld.version}</version>
    <scope>runtime</scope>
</dependency>
...

文件 src/main/webapp/WEB-INF/beans.xml 仅包含用于激活 CDI 的空 beans 元素:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/beans_1_0.xsd">
</beans>

文件 scr/main/webapp/WEB-INF/jetty-env.xml 包含用于通过 JNDI 获取 bean 管理器的 Jetty 特定配置:
<!DOCTYPE Configure PUBLIC "-//Mort Bay Consulting//DTD Configure//EN" "http://www.eclipse.org/jetty/configure.dtd">

<Configure id="webAppCtx" class="org.eclipse.jetty.webapp.WebAppContext">
    <New id="BeanManager" class="org.eclipse.jetty.plus.jndi.Resource">
        <Arg>
            <Ref id="webAppCtx" />
        </Arg>
        <Arg>BeanManager</Arg>
        <Arg>
            <New class="javax.naming.Reference">
                <Arg>javax.enterprise.inject.spi.BeanManager</Arg>
                <Arg>org.jboss.weld.resources.ManagerObjectFactory</Arg>
                <Arg />
            </New>
        </Arg>
    </New>
</Configure>

文件 src/main/webapp/WEB-INF/web.xml 包含用于引导 CDI 的 servlet 监听器和 BeanManager 引用:
...
<listener>
    <listener-class>org.jboss.weld.environment.servlet.Listener</listener-class>
</listener>

<resource-env-ref>
    <description>Object factory for the CDI Bean Manager</description>
    <resource-env-ref-name>BeanManager</resource-env-ref-name>
    <resource-env-ref-type>javax.enterprise.inject.spi.BeanManager</resource-env-ref-type>
</resource-env-ref>
...

通过这些设置，我在使用 运行应用程序时总是遇到以下错误mvn jetty :运行 (第一行最重要):
2012-07-31 11:09:33,546 [main] ERROR org.jboss.weld.environment.jetty.JettyPost72Container - Unable to create JettyWeldInjector. CDI injection will not be available in Servlets, Filters or Listeners
java.lang.IllegalArgumentException: Cannot load class for org.jboss.weld.environment.jetty.WeldDecorator
    at org.jboss.weld.environment.servlet.util.Reflections.classForName(Reflections.java:58)
    at org.jboss.weld.environment.jetty.JettyPost72Container.initialize(JettyPost72Container.java:66)
    at org.jboss.weld.environment.servlet.Listener.contextInitialized(Listener.java:162)
    at org.eclipse.jetty.server.handler.ContextHandler.callContextInitialized(ContextHandler.java:764)
    at org.eclipse.jetty.servlet.ServletContextHandler.callContextInitialized(ServletContextHandler.java:406)
    at org.eclipse.jetty.server.handler.ContextHandler.startContext(ContextHandler.java:756)
    at org.eclipse.jetty.servlet.ServletContextHandler.startContext(ServletContextHandler.java:242)
    at org.eclipse.jetty.webapp.WebAppContext.startContext(WebAppContext.java:1234)
    at org.eclipse.jetty.server.handler.ContextHandler.doStart(ContextHandler.java:699)
    at org.eclipse.jetty.webapp.WebAppContext.doStart(WebAppContext.java:467)
    at org.mortbay.jetty.plugin.JettyWebAppContext.doStart(JettyWebAppContext.java:256)
    at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:59)
    at org.eclipse.jetty.server.handler.HandlerCollection.doStart(HandlerCollection.java:224)
    at org.eclipse.jetty.server.handler.ContextHandlerCollection.doStart(ContextHandlerCollection.java:167)
    at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:59)
    at org.eclipse.jetty.server.handler.HandlerCollection.doStart(HandlerCollection.java:224)
    at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:59)
    at org.eclipse.jetty.server.handler.HandlerWrapper.doStart(HandlerWrapper.java:90)
    at org.eclipse.jetty.server.Server.doStart(Server.java:262)
    at org.mortbay.jetty.plugin.JettyServer.doStart(JettyServer.java:65)
    at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:59)
    at org.mortbay.jetty.plugin.AbstractJettyMojo.startJetty(AbstractJettyMojo.java:511)
    at org.mortbay.jetty.plugin.AbstractJettyMojo.execute(AbstractJettyMojo.java:364)
    at org.mortbay.jetty.plugin.JettyRunMojo.execute(JettyRunMojo.java:516)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:101)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:209)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:153)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:145)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:84)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:59)
    at org.apache.maven.lifecycle.internal.LifecycleStarter.singleThreadedBuild(LifecycleStarter.java:183)
    at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:161)
    at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:320)
    at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:156)
    at org.apache.maven.cli.MavenCli.execute(MavenCli.java:537)
    at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:196)
    at org.apache.maven.cli.MavenCli.main(MavenCli.java:141)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:290)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:230)
    at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:409)
    at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:352)
Caused by: java.lang.NoClassDefFoundError: org/eclipse/jetty/servlet/ServletContextHandler$Decorator
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
    at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at org.eclipse.jetty.webapp.WebAppClassLoader.loadClass(WebAppClassLoader.java:415)
    at org.eclipse.jetty.webapp.WebAppClassLoader.loadClass(WebAppClassLoader.java:377)
    at org.jboss.weld.environment.servlet.util.Reflections.classForName(Reflections.java:51)
    ... 44 more
Caused by: java.lang.ClassNotFoundException: org.eclipse.jetty.servlet.ServletContextHandler$Decorator
    at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at org.eclipse.jetty.webapp.WebAppClassLoader.loadClass(WebAppClassLoader.java:415)
    at org.eclipse.jetty.webapp.WebAppClassLoader.loadClass(WebAppClassLoader.java:377)
    ... 56 more

应用程序正在运行，但异常消息指出:CDI 注入(inject)在 Servlet 中将不可用。

几天前，我发现了这篇文章Starting a CDI webapp from Maven with Weld-Servlet and Jetty Plugin尤其是 部分允许注入(inject) servlet 的最后一个技巧 .作者解释说 Weld 需要装饰一些 Jetty 内部类才能在 servlet 中使用 @Inject 注释，并且我们必须告诉 Jetty 类加载器在哪里可以找到这个类，以便让 Weld servlet 装饰它。

我等了这么久才得到这个特殊的信息。所以我添加了文件 src/main/webapp/WEB-INF/jetty-context.xml :
<!DOCTYPE Configure PUBLIC "-//Jetty//Configure//EN" "http://www.eclipse.org/jetty/configure.dtd">

<Configure id="webAppCtx" class="org.eclipse.jetty.webapp.WebAppContext">
    <Set name="serverClasses">
        <Array type="java.lang.String">
            <Item>org.eclipse.jetty.servlet.ServletContextHandler.Decorator</Item>
        </Array>
    </Set>
</Configure>

我也更新了 pom.xml 为了合并文件 jetty-context.xml(最后一行):
...
<plugin>
    <groupId>org.mortbay.jetty</groupId>
    <artifactId>jetty-maven-plugin</artifactId>
    <version>${jetty.version}</version>
    <configuration>
        <webApp>
            <contextPath>/${project.build.finalName}</contextPath>
        </webApp>
        <jettyEnvXml>src/main/webapp/WEB-INF/jetty-env.xml</jettyEnvXml>
        <contextXml>src/main/webapp/WEB-INF/jetty-context.xml</contextXml>
        ...

现在上面的错误消失了。在那短暂的时刻我很高兴:)

在 src/main/resources/log4j.properties 中，我增加了 Weld 的日志级别:
...
log4j.logger.org.jboss.weld=DEBUG, stdout
log4j.additivity.org.jboss.weld=false
...

启动应用程序时，我收到以下焊接日志消息:
...
2012-07-31 12:00:08,968 [main] INFO  org.jboss.weld.Bootstrap - WELD-000101 Transactional services not available. Injection of @Inject UserTransaction not available. Transactional observers will be invoked synchronously.
2012-07-31 12:00:09,109 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000103 Enabled alternatives for Manager
Enabled alternatives: [] []
Registered contexts: [interface javax.enterprise.context.SessionScoped, interface javax.enterprise.context.ApplicationScoped, interface javax.enterprise.context.Dependent, interface javax.enterprise.context.RequestScoped, interface javax.enterprise.context.ConversationScoped, interface javax.inject.Singleton]
Registered beans: 0
: [] []
2012-07-31 12:00:09,109 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000104 Enabled decorator types for Manager
Enabled alternatives: [] []
Registered contexts: [interface javax.enterprise.context.SessionScoped, interface javax.enterprise.context.ApplicationScoped, interface javax.enterprise.context.Dependent, interface javax.enterprise.context.RequestScoped, interface javax.enterprise.context.ConversationScoped, interface javax.inject.Singleton]
Registered beans: 0
: []
2012-07-31 12:00:09,109 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000105 Enabled interceptor types for Manager
Enabled alternatives: [] []
Registered contexts: [interface javax.enterprise.context.SessionScoped, interface javax.enterprise.context.ApplicationScoped, interface javax.enterprise.context.Dependent, interface javax.enterprise.context.RequestScoped, interface javax.enterprise.context.ConversationScoped, interface javax.inject.Singleton]
Registered beans: 0
: []
2012-07-31 12:00:09,171 [main] INFO  org.jboss.weld.environment.jetty.JettyPost72Container - Jetty7 detected, JSR-299 injection will be available in Listeners, Servlets and Filters.
2012-07-31 12:00:09,265 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: Implicit Bean [javax.enterprise.inject.spi.InjectionPoint] with qualifiers [@Default]
2012-07-31 12:00:09,265 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: org.jboss.weld.bean-flat-Built-in-org.jboss.weld.context.DependentContext
2012-07-31 12:00:09,281 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: Managed Bean [class de.telexiom.cardiom.util.MediaType] with qualifiers [@Any @Default]
2012-07-31 12:00:09,281 [main] WARN  org.jboss.weld.interceptor.util.InterceptionTypeRegistry - Class 'javax.ejb.PostActivate' not found, interception based on it is not enabled
2012-07-31 12:00:09,281 [main] WARN  org.jboss.weld.interceptor.util.InterceptionTypeRegistry - Class 'javax.ejb.PrePassivate' not found, interception based on it is not enabled
2012-07-31 12:00:09,281 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: Managed Bean [class de.telexiom.cardiom.web.LoginBean] with qualifiers [@Any @Default]
2012-07-31 12:00:09,281 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: org.jboss.weld.bean-flat-Built-in-org.jboss.weld.context.http.HttpRequestContext
2012-07-31 12:00:09,281 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: org.jboss.weld.bean-flat-Built-in-org.jboss.weld.context.http.HttpSessionContext
2012-07-31 12:00:09,281 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: Managed Bean [class de.telexiom.cardiom.web.CustomerBean] with qualifiers [@Any @Default @Named]
2012-07-31 12:00:09,281 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: org.jboss.weld.bean-flat-Built-in-org.jboss.weld.context.bound.BoundConversationContext
2012-07-31 12:00:09,281 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: org.jboss.weld.bean-flat-Built-in-org.jboss.weld.context.ApplicationContext
2012-07-31 12:00:09,281 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: Managed Bean [class de.telexiom.cardiom.util.LocaleBean] with qualifiers [@Any @Default @Named]
2012-07-31 12:00:09,281 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: org.jboss.weld.bean-flat-Built-in-org.jboss.weld.context.bound.BoundRequestContext
2012-07-31 12:00:09,281 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: org.jboss.weld.bean-flat-Built-in-org.jboss.weld.context.SingletonContext
2012-07-31 12:00:09,281 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: Implicit Bean [javax.enterprise.inject.Instance] with qualifiers [@Default]
2012-07-31 12:00:09,296 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: Managed Bean [class de.telexiom.cardiom.DataReader] with qualifiers [@Any @Default @Named]
2012-07-31 12:00:09,296 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: Managed Bean [class de.telexiom.cardiom.SessionBean] with qualifiers [@Any @Default @Named]
2012-07-31 12:00:09,296 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: Managed Bean [class de.telexiom.cardiom.ConnectionTable] with qualifiers [@Any @Default @Named]
2012-07-31 12:00:09,296 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: org.jboss.weld.bean-flat-Built-in-org.jboss.weld.context.http.HttpConversationContext
2012-07-31 12:00:09,296 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: Managed Bean [class de.telexiom.cardiom.servlet.ClientServlet] with qualifiers [@Any @Default]
2012-07-31 12:00:09,296 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: org.jboss.weld.bean-flat-Built-in-org.jboss.weld.context.RequestContext
2012-07-31 12:00:09,296 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: org.jboss.weld.bean-flat-Built-in-javax.enterprise.context.Conversation
2012-07-31 12:00:09,296 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: Implicit Bean [javax.enterprise.event.Event] with qualifiers [@Default]
2012-07-31 12:00:09,296 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: Managed Bean [class de.telexiom.cardiom.CustomerListBean] with qualifiers [@Any @Default]
2012-07-31 12:00:09,296 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: Managed Bean [class de.telexiom.cardiom.Customer] with qualifiers [@Any @Default]
2012-07-31 12:00:09,296 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: Managed Bean [class de.telexiom.cardiom.servlet.StreamingServlet] with qualifiers [@Any @Default]
2012-07-31 12:00:09,296 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: org.jboss.weld.bean-flat-Built-in-org.jboss.weld.context.bound.BoundSessionContext
2012-07-31 12:00:09,296 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000106 Bean: Managed Bean [class de.telexiom.cardiom.SessionInfo] with qualifiers [@Any @Default]
2012-07-31 12:00:09,312 [main] DEBUG org.jboss.weld.Reflection - WELD-000601 interface javax.inject.Named is missing @Target. Weld will use this annotation, however this may make the application unportable.
2012-07-31 12:00:09,312 [main] DEBUG org.jboss.weld.Bootstrap - WELD-000100 Weld initialized. Validating beans
...

我使用 Servlet 3.0 API , StreamingServlet 和 ClientServlet 通过注解配置。问题是应用程序作用域 bean 连接表和 数据读取器未注入(inject):
@WebServlet(value = "/stream", initParams = @WebInitParam(name = "maxIdleTime", value = "0"))
public class StreamingServlet extends HttpServlet
{
    @Inject
    private ConnectionTable connectionTable;
    ...

@WebServlet(value = "/push", loadOnStartup = 1, initParams = @WebInitParam(name = "maxIdleTime", value = "0"))
public class ClientServlet extends WebSocketServlet
{
    @Inject
    private ConnectionTable connectionTable;

    @Inject
    private DataReader dataReader;
    ...

调用(未)注入(inject)的属性，我总是得到 NullPointerExceptions。通过 web.xml 配置 Servlet 时，问题也仍然存在。我对我做错了什么没有更多的想法。结合 Jetty 和 Maven 阅读所有关于 CDI 的文章，我想知道我是否是唯一一个遇到这些注入(inject)问题的人。

在我的 JSF bean 类中 UserBean.java 注入(inject)有效，我不必使用 setter 方法来初始化属性 connectionTable 和 dataReader。

在 DataReader 构造函数(以及 ConnectionTable 类)中使用下面的输出语句
@ApplicationScoped
public class DataReader implements Serializable
{
    ...
    public DataReader()
    {
        System.out.println("DataReader.DataReader(): " + this);
    }
    ...

我想知道标准输出上显示的 2 个 DataReader 实例:
...
2012-07-31 14:03:04,843 [qtp5435124-17] DEBUG org.jboss.weld.JSF - WELD-000504 Resuming conversation with id null
31.07.2012 14:03:05 org.apache.myfaces.util.ExternalSpecifications isUnifiedELAvailable
INFO: MyFaces Unified EL support enabled
DataReader.DataReader(): DataReader$Proxy$_$$_WeldClientProxy@506e6d
ConnectionTable.ConnectionTable(): ConnectionTable$Proxy$_$$_WeldClientProxy@93b16c

2012-07-31 14:03:19,093 [qtp5435124-16] DEBUG org.jboss.weld.JSF - WELD-000504 Resuming conversation with id null
DataReader.DataReader(): DataReader@1cbd382

前 5 行在基于表单的身份验证后打印，最后 2 行出现在第一次处理我的 JSF 操作时使用注入(inject)的 dataReader 属性。由于 bean 是应用程序范围的，我真的很想知道 2 个构造函数传递。但是从应用程序的行为来看，我认为它没有任何问题。

我的最后一个假设是 servlet 以某种方式存在于分离的、独立的上下文或容器或类似的东西中。因为也无法在其他 2 个 Servlet 中获取 FacesContext: FacesContext.getCurrentInstance() 给 空 .但我不知道这在某种程度上是否重要。

我希望任何人都可以帮助我找出问题所在。先感谢您

塞巴斯蒂安

最佳答案

错误很明显:

java.lang.NoClassDefFoundError: org/eclipse/jetty/servlet/ServletContextHandler$Decorator

Weld 找不到上面的 Jetty 服务器类。之所以找不到这个类，是因为Jetty的类加载机制。 Jetty 的 WebAppClassloader 有一个默认的服务器类列表，它不会加载:见 http://wiki.eclipse.org/Jetty/Reference/Jetty_Classloading
Server Classes
-org.eclipse.jetty.continuation.             don't hide continuation classes
-org.eclipse.jetty.jndi.                     don't hide naming classes
-org.eclipse.jetty.plus.jaas.                don't hide jaas classes
-org.eclipse.jetty.websocket.                don't hide websocket extension
-org.eclipse.jetty.servlet.DefaultServlet    don't hide default servlet
org.eclipse.jetty.                           hide all other jetty classes

要允许 Jetty 加载此类，您需要添加“-org.eclipse.jetty.servlet”。到 jetty-web.xml 中的服务器类列表。
 <Configure class="org.eclipse.jetty.webapp.WebAppContext">
   <Call name="prependServerClass"><Arg>-org.eclipse.jetty.servlet.</Arg></Call>
 </Configure>

或者，如果您使用的是嵌入式 jetty ，则调用以下方法:
 context.prependServerClass("-org.eclipse.jetty.servlet.");

关于jakarta-ee - CDI 注入(inject)在 Servlet 中不起作用，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/11740322/

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