从jar加载类

Question

This is the common approach with working with loading class dynamically:

try {
        File file = new File(JAR_FILE);
        String classToLoad = "com.mycompany.MyClass";
        URL jarUrl = new URL("jar", "","file:" + file.getAbsolutePath()+"!/");
        URLClassLoader loader = new URLClassLoader(new URL[] {jarUrl}, Thread.currentThread().getContextClassLoader());
        Class c = loader.loadClass(classToLoad); 
    } catch (Exception e) {
        e.printStackTrace();
    }

However, I need an approach where:

• We don't need to create a File (as the jar I am trying to process is a byte array[] when fetched)
• Or we won't need to create a temporary file from byte[] array (as AppEngine, the platform I work with does not allow to create temporary files)

Answer 1

You will have to create your own class loader.

Something like this, basic idea in pseudo code:

 class MyClassLoader extends ClassLoader {
     public Class findClass(String name) {
         byte[] b = loadClassData(name);
         return defineClass(name, b, 0, b.length);
     }

     private byte[] loadClassData(String name) {
         JarInputStream jis = new JarInputStream(new ByteArrayInputStream(bytearrayJarData));
         JarEntry entry = jis.getNextJarEntry();
         while (entry != null) {
                //compare entry to requested class
                // if match, return Byte data
                // else entry = jis.getNextJarEntry();
         }
         return null; // nothing found
     }
 }

Answer 2

Write your own ClassLoader and override findClass() . There you can use defineClass() to load your byte[] .