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URLClassLoader未加载jar文件

[英]URLClassLoader not loading jar file

 原文  2019-03-04 15:53:56  2   1    java

问题描述

I'm currently working on an web application, that must load jar files from a directory on the local drive. 我当前正在使用Web应用程序，该应用程序必须从本地驱动器上的目录加载jar文件。 After a bit of researching, the URLClassLoaders seems to be the best fitting solution. 经过一些研究， URLClassLoaders似乎是最合适的解决方案。

The beef: 牛肉： 

public @Slf4j class ActionLoader {
private static void addFilesToClassPath(File path) {
        URLClassLoader clazzLoader = null;
        try {
            clazzLoader = new URLClassLoader(new URL[] {path.toURI().toURL()},
                                         ActionLoader.class.getClassLoader());
            try {
                Class.forName("de.befoe.xyz.ActionTest", false, clazzLoader);
            } catch (ClassNotFoundException e) {
                log.error("Class not found", e);
            }
            try {
                clazzLoader.loadClass("de.befoe.xyz.ActionTest");
            } catch (ClassNotFoundException e) {
                log.error("Class not found", e);
            }
        } catch (SecurityException e) {
            log.error("Access denied", e);
        } catch (MalformedURLException e) {
            log.error("Malformed URL", e);
        } finally {
            if (clazzLoader != null) {
                try {
                    clazzLoader.close();
                } catch (IOException e) {
                    log.error("Error closing the class loader", e);
                }
            }
        }
    }
}

First ClassNotFoundException : First ClassNotFoundException ： 

java.lang.ClassNotFoundException: de.befoe.xyz.ActionTest
    at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    at de.procrm.speedindexer.scr.ActionLoader.addFilesToClassPath(ActionLoader.java:80)
    at de.procrm.speedindexer.scr.ActionLoader.loadActions(ActionLoader.java:31)
    at de.procrm.speedindexer.scr.Init.contextInitialized(Init.java:39)
    at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4842)
    at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5303)
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:147)
    at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:725)
    at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:701)
    at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:717)
    at org.apache.catalina.startup.HostConfig.deployWAR(HostConfig.java:940)
    at org.apache.catalina.startup.HostConfig.deployApps(HostConfig.java:467)
    at org.apache.catalina.startup.HostConfig.check(HostConfig.java:1612)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.apache.tomcat.util.modeler.BaseModelMBean.invoke(BaseModelMBean.java:300)
    at com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.invoke(DefaultMBeanServerInterceptor.java:819)
    at com.sun.jmx.mbeanserver.JmxMBeanServer.invoke(JmxMBeanServer.java:801)
    at org.apache.catalina.manager.ManagerServlet.check(ManagerServlet.java:1451)
    at org.apache.catalina.manager.ManagerServlet.deploy(ManagerServlet.java:724)
    at org.apache.catalina.manager.ManagerServlet.doPut(ManagerServlet.java:423)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:651)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.catalina.filters.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:108)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:614)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:616)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:528)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1099)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:670)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1520)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1476)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:748)

Second ClassNotFoundException : 第二类ClassNotFoundException ： 

java.lang.ClassNotFoundException: de.befoe.xyz.ActionTest
    at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:348)
    at de.procrm.speedindexer.scr.ActionLoader.addFilesToClassPath(ActionLoader.java:81)
    at de.procrm.speedindexer.scr.ActionLoader.loadActions(ActionLoader.java:31)
    at de.procrm.speedindexer.scr.Init.contextInitialized(Init.java:39)
    at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4842)
    at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5303)
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:147)
    at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:725)
    at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:701)
    at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:717)
    at org.apache.catalina.startup.HostConfig.deployWAR(HostConfig.java:940)
    at org.apache.catalina.startup.HostConfig.deployApps(HostConfig.java:467)
    at org.apache.catalina.startup.HostConfig.check(HostConfig.java:1612)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.apache.tomcat.util.modeler.BaseModelMBean.invoke(BaseModelMBean.java:300)
    at com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.invoke(DefaultMBeanServerInterceptor.java:819)
    at com.sun.jmx.mbeanserver.JmxMBeanServer.invoke(JmxMBeanServer.java:801)
    at org.apache.catalina.manager.ManagerServlet.check(ManagerServlet.java:1451)
    at org.apache.catalina.manager.ManagerServlet.deploy(ManagerServlet.java:724)
    at org.apache.catalina.manager.ManagerServlet.doPut(ManagerServlet.java:423)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:651)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.catalina.filters.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:108)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:614)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:616)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:528)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1099)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:670)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1520)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1476)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:748)

The path-variable points to the directory containing the jar files (I also tried initializing the class loader with URLs pointing to the jar files directly). 路径变量指向包含jar文件的目录（我也尝试使用直接指向jar文件的URL初始化类加载器）。

When I now try to access a class from one of the jar files, I get a ClassNotFoundException . 现在，当我尝试从一个jar文件访问一个类时，出现ClassNotFoundException 。 When inspecting the class loader in the debugger, I can also see, that the packages- and the classes-fields are both empty, so it doesn't seem as the class loader did load anything. 在调试器中检查类加载器时，我还可以看到，packages和classes-字段均为空，因此看起来好像类加载器没有加载任何内容。

First I thought it has something to do with missing dependencies, so I created a second jar file with no dependencies and added the classes parent class loader to the constructor, but it changed nothing. 首先，我认为这与缺少依赖项有关，因此我创建了一个没有依赖项的第二个jar文件，并将类的父类加载器添加到构造函数中，但是它什么也没有改变。

Ideas? 有想法吗？

E: After a bit of researching it seems my first thought (dependencies) was correct. E：经过一番研究，似乎我的第一个想法（依赖性）是正确的。 I have another library that is used by the web application and the loaded jar files. 我有另一个供Web应用程序和加载的jar文件使用的库。 If I add this library to the the list of jars to add to the URLClassLoader, the jars are loaded just fine. 如果我将此库添加到要添加到URLClassLoader的jar列表中，则可以很好地加载jar。 But sure, this is not, what I want to do, because technically my web app uses another class than my jar files, what leads to other problems. 但是可以肯定的是，这不是我想要做的，因为从技术上讲，我的Web应用程序使用的不是jar文件，而是其他类，这会导致其他问题。

The question now is why the parent class loaders classes are not used properly. 现在的问题是为什么父类加载器类未正确使用。 After further research it seems the parent class loader I used by calling ActionLoader.class.getClassLoader() was not the correct one. 经过进一步研究，似乎我通过调用ActionLoader.class.getClassLoader（）使用的父类加载器不是正确的加载器。 In eclipse debugger therefore I saw a field called delegate which was set to false. 因此，在eclipse调试器中，我看到了一个称为委托的字段，该字段设置为false。 So I instead use this class loaders parent, and the files are loading just fine. 因此，我改为使用此类的父级加载器，并且文件加载也很好。

1 个解决方案

解决方案1
 0  2019-03-04 16:22:23

I don't have your JAR file so i've tested with one of my JAR files on a custom path, and the class is correctly loaded, using the systemClassLoader. 我没有您的JAR文件，因此我已经在自定义路径上对我的一个JAR文件进行了测试，并且使用systemClassLoader正确加载了该类。

Here is the sample code: 这是示例代码： 

addFilesToClassPath(new File("C:/Users/Domenico/.m2/repository/com/doctorchat/core/0.1.ALPHA/core-0.1.ALPHA.jar"));

private static void addFilesToClassPath(File path) {

   try(URLClassLoader clazzLoader = new URLClassLoader(new URL[]{path.toURI().toURL()}, ClassLoader.getSystemClassLoader())) {
            Class c = clazzLoader.loadClass("com.doctorchat.core.exceptions.NotImplementedException");
            System.out.println(c);
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        //  clazzLoader.getResource()

    }

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