为什么我不能通过ExtClassLoader加载类？

Question

This is the structure of my Java project:

package: com.study.test
              ---- HelloWorld.java
              ---- Test.java

The content of HelloWorld.java :

package com.study.test;

public class HelloWorld {}

The content of Test.java :

package com.study.test;

public class Test {

    public static void main(String[] args) {
        System.out.println(System.getProperty("java.ext.dirs"));
        System.out.println(HelloWorld.class.getClassLoader());
    }

}

The value of the " java.ext.dirs " is:

D:\glassfish4\jdk7\jre\lib\ext;C:\windows\Sun\Java\lib\ext

I put the class file HelloWorld.class in directory:

D:\glassfish4\jdk7\jre\lib\ext\com\study\test

but the output of " System.out.println(HelloWorld.class.getClassLoader()); " is:

sun.misc.Launcher$AppClassLoader@70a0afab

Why the class HelloWorld isn't loaded by ExtClassLoader? ExtClassLoader is the parent class loader of AppClassLoader.

Answer 1

The extension classloader load only classes from within Jar files.

assume following structure

src/Test.java
src/HelloWorld.java

amend Test.java as ( edit this is not really necessary, only to show that the resolving is not done during compile time)

package com.study.test;
public class Test {
    public static void main(String[] args) throws Exception {
        System.out.println(System.getProperty("java.ext.dirs"));
        System.out.println(Class.forName("com.study.test.HelloWorld")
            .getClassLoader());
    }
}

compile them

mkdir bin/
javac -d bin/ src/Test.java src/HelloWorld.java

create a Jar file containing the classes

jar cf test-hello.jar -C bin/ .

verify the content of the Jar file

jar tf test-hello.jar

output

META-INF/                       
META-INF/MANIFEST.MF            
com/                            
com/study/                      
com/study/test/                 
com/study/test/HelloWorld.class 
com/study/test/Test.class       

move the test-hello.jar in the extension directory

mv test-hello.jar D:\glassfish4\jdk7\jre\lib\ext

run your class Test

java com.study.test.Test

output similar (the directories where copied from your questin)

D:\glassfish4\jdk7\jre\lib\ext;C:\windows\Sun\Java\lib\ext
sun.misc.Launcher$ExtClassLoader@70dea4e