ClassNotFoundException while loading a class from file with URLClassLoader

问题

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) {

    }
}

I am getting the following exception.

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)

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

The class which is loaded looks as follows.

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.");
    }
}

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

回答1:

Problem:

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

Solution:

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

回答2:

try passing the caller's classloader as a parameter in the constructor:

URLClassLoader classLoader = new URLClassLoader(urls, getClass().class.getClassLoader());

the class is a direct child of the dir in the url, so you need no package:

Target target =
 (Target)classLoader.
 loadClass("OdbcIniTarget").
 newInstance();

来源：https://stackoverflow.com/questions/24685807/classnotfoundexception-while-loading-a-class-from-file-with-urlclassloader

标签

java

reflection

urlclassloader