运行 java -jar 时出现 java.lang.ClassNotFoundException [英] java.lang.ClassNotFoundException when running java -jar

问题描述

我正在使用 ant 来构建我的 build.xml 文件，它编译正常，但是在通过运行生成的 jar 时获得运行时 java.lang.NoClassDefFoundErrorjava -jar my_jar.jar".似乎这种情况经常出现，但没有一个相关问题的解决方案对我有用.

I'm using ant to build my build.xml file, it compiles ok, but then getting a runtime java.lang.NoClassDefFoundError when running the resulting jar via "java -jar my_jar.jar". It seems like this comes up a lot but none of the related questions' solutions worked for me.

我的 javac 类路径只包含/usr/local/lib/libthrift.jar"，主 .java 文件导入了一个一堆 thrift 包，例如 org.apache.thrift.transport.TTransportException.

My classpath for javac contains only "/usr/local/lib/libthrift.jar" and the main .java file imports a bunch of thrift packages such as org.apache.thrift.transport.TTransportException.

当我尝试通过以下方式运行程序时:

When I try running the program via:

java -jar MyClass.jar

，我收到错误:

Exception in thread "main" **java.lang.NoClassDefFoundError**: org/apache/thrift/transport/TTransportException
Caused by: java.lang.ClassNotFoundException: org.apache.thrift.transport.TTransportException
        at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:252)
        at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:320)
Could not find the main class: **MyClass**. Program will exit.

以下是我迄今为止尝试过但不起作用的方法:

Here are the things I've tried so far that don't work:

• 在命令行上添加一个标志，如java -cp/usr/local/lib/libthrift.jar -jar my_jar.jar"，结果和上面一样报错

• adding a flag on the command line like "java -cp /usr/local/lib/libthrift.jar -jar my_jar.jar", the result is the same error as above

添加<attribute name="Class-Path" value="./:/usr/local/lib/libthrift.jar"/> 在我的 jar 的 manifest> 标签中，结果是和上面一样的错误

adding <attribute name="Class-Path" value="./:/usr/local/lib/libthrift.jar"/> inside my jar's manifest> tag, the result is the same error as above

将 -Xbootclasspath/a:/usr/local/lib/libthrift.jar:./ 添加到 java 命令行.它解决了第一个错误，但出现了另一个错误:

adding -Xbootclasspath/a:/usr/local/lib/libthrift.jar:./ to the java command line. it solves the first error but a different error comes up:

线程main"中的异常 java.lang.NoClassDefFoundError: org/apache/log4j/Logger在 org.apache.thrift.transport.TServerSocket.<clinit>(TServerSocket.java:36)在 MyClass.start(来源不明)在 MyClass.main(来源不明)

Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/log4j/Logger at org.apache.thrift.transport.TServerSocket.<clinit>(TServerSocket.java:36) at MyClass.start(Unknown Source) at MyClass.main(Unknown Source)

如果我注释掉实例化缺失类但保留导入的代码，则代码执行正常.

If I comment out the code that instantiates the missing classes but leave the imports, the code executes fine.

我将我的 java 类移到了服务器，并在 manifest 属性中引用了带有服务器的 MainClass，但这并没有解决任何问题.

I moved my java classes to a server and referenced the MainClass with the server in the manifest attribute, but that didn't fix anything.

推荐答案

Could not find the main class: MyClass

该错误似乎实际上与您的 MANIFEST 有关:

The error seems actually related to your MANIFEST which:

• 可能没有完整的类路径 Class-Path:参见 此操作方法

当您有 jar 时，最好的解决方案是尝试将所需的 jar 包含到清单声明中.

The best solution when you have a jar is to try to include the required jars into the manifest declaration.

Manifest-Version: 1.0 
Class-Path:  
 customer_client.jar  
 mailer_client.jar  
 signon_client.jar 

• 或者可能没有在您的my_jar.jar"中充分定义 MainClass.

请参阅此操作方法:

<target name="jar" depends="compile">
     <delete file="hello.jar"/>
     <delete file="MANIFEST.MF"/>
     <manifest file="MANIFEST.MF">
        <attribute name="Built-By" value="${user.name}"/>
        <attribute name="Main-Class" value="howto.Hello"/>
    </manifest>

      <jar destfile="hello.jar"
           basedir="."
           includes="**/*.class"
           manifest="MANIFEST.MF"
           />
  </target>

<attribute name="Main-Class" value="howto.Hello"/> 需要指定MainClass的完整路径(包)，而不仅仅是 MainClass.

the <attribute name="Main-Class" value="howto.Hello"/> needs to specify the full path (packages) of the MainClass, not just MainClass.

如果您的主类在默认包中(未命名包)，我不确定它是否可以被加载程序引用(请参阅此SO 问题)
因此，将您的 JarRunner 移动到一个包中，并在 <attribute name="Main-Class" value="myPackage.JarRunner"/> 元素中适当地声明它.

If your main class is in the default package (the unnamed package), I am not sure it can be referenced by the loader (see this SO question)
So move your JarRunner into a package, and declare it appropriately in the <attribute name="Main-Class" value="myPackage.JarRunner"/> element.

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