使用URLClassLoader从文件加载类时发生ClassNotFoundException [英] ClassNotFoundException while loading a class from file with URLClassLoader

问题描述

我正在使用以下代码创建实现Target接口的OdbcIniTarget类的实例. OdbcIniTarget类已存在于应用程序中，但位于不同的程序包中，现在应从不在类路径中的 target 目录中加载.

I am using the following code to create an instance of OdbcIniTarget class which implements the Target interface. The OdbcIniTarget class already exists in the application but is located in a different package and now should be loaded from the target directory which is not in the class path.

...
// directory below is resolved to /D:/Workspace/<myproject>/targets/
private File targetsLocation = new File("targets");
...
private void loadTargets() {            

    URL url = null;
    try {
        url = targetsLocation.toURI().toURL();            
    } catch (MalformedURLException e) {

    }

    URL[] urls = new URL[]{ url };        
    URLClassLoader classLoader = new URLClassLoader(urls);

    try {

     Target target =
     (Target)classLoader.
     loadClass("ch.blah.targets.nope.OdbcIniTarget").
     newInstance(); // <--- this fails hard  

    } catch (...) {
     ...
    }            

    try {
        classLoader.close();
    } catch (IOException e) {

    }
}

我遇到以下异常.

java.lang.ClassNotFoundException: ch.blah.targets.nope.OdbcIniTarget
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at ch.blah.targets.TargetFactory.loadTargets(TargetFactory.java:74)
    at ch.blah.targets.TargetFactory.<init>(TargetFactory.java:41)
    at ch.blah.SomeOfMyClasses.<init>(SomeOfMyClasses.java:34)
    at ch.blah.TheMainClass.main(TheMainClass.java:38)

有人知道为什么找不到该类吗?

Does someone have an idea why the class is not found?

所加载的类如下.

package ch.blah.targets.nope;

import org.apache.log4j.Logger;

import ch.blah.utils.Utils;

public class OdbcIniTarget extends Target {

    private static final Logger log = Logger.getLogger(OdbcIniTarget.class);    

    @Override
    public boolean someMethod1() {
        return false;
    }

    @Override
    public boolean someMethod2() {
        return false;
    }

    @Override
    public boolean someMethod3() {
        return false;
    }       

    @Override
    public void someMethod4() {
        log.debug("Blah.");
        Utils.getInstance().marshalClass(SomeClass.class);
        log.debug("Finished.");
    }
}

目标 目录的目录结构如下.

The directory structure for targets directory is as follows.

D:\Workspace\<myproject>\targets>dir
 Volume in drive D is DATA
 Volume Serial Number is 021C-EC9B

 Directory of D:\Workspace\<myproject>\targets

10.07.2014  21:20    <DIR>          .
10.07.2014  21:20    <DIR>          ..
10.07.2014  21:31             1'278 OdbcIniTarget.java
10.07.2014  20:23             3'761 OdbcIniTargetConfiguration.java
               2 File(s)          5'039 bytes
               2 Dir(s)   7'328'571'392 bytes free

推荐答案

问题:

无法加载，因为它不包含该URL上的类，它具有Java源代码

It is failing to load because it doesn't contain classes at that URL, it has java source

解决方案:

将这些Java类编译为.class文件，然后将url传递到看起来像这样的目录中

Compile those java classes to .class file and then pass the url to the directory which should look like

ch\blah\targets\nope\OdbcIniTarget.class

这篇关于使用URLClassLoader从文件加载类时发生ClassNotFoundException的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！