ClassLoaderDemo1public class ClassLoaderDemo1 extends ClassLoader | Demonstration of a ClassLoader |
| Fields Summary |
|---|
| protected Hashtable | cacheThe Hashtable to keep track of classes, to avoid re-loading them | | private final int | dataLengthdata's expected length | | private int[] | datadata, obtained by dumping a compiled .class file |
| Methods Summary |
|---|
| private byte[] | genClassData(java.lang.String name)"load", that is, make up, the data for the class
if (dataLength != data.length)
throw new IllegalArgumentException("data corrupt");
byte[] bd = new byte[data.length];
for (int i=0; i<bd.length; i++)
bd[i] = (byte)data[i];
return bd;
| | public synchronized java.lang.Class | loadClass(java.lang.String name, boolean resolve)
/** We can expect to be called to resolve at least demo's
* superclass (java.lang.Object). Fortunatetely, we can just
* use super.findSystemClass() to load such things...
*/
if (name.startsWith("java.")) {
System.out.println("loadClass: SystemLoading " + name);
return findSystemClass(name);
}
Class c = (Class)cache.get(name);
if (c == null) {
System.out.println("loadClass: About to genClassData " + name);
byte mydata[] = genClassData(name);
System.out.println("loadClass: About to defineClass " + name);
c = defineClass(name, mydata, 0, mydata.length);
System.out.println("loadClass: storing " + name + " in cache.");
cache.put(name, c);
} else
System.out.println("loadClass: found " + name + " in cache.");
if (resolve) {
System.out.println("loadClass: About to resolveClass " + name);
resolveClass(c);
}
return c;
| | public static void | main(java.lang.String[] argv)
System.out.println("ClassLoaderDemo1 starting");
ClassLoaderDemo1 loader = new ClassLoaderDemo1();
Class c = null;
Object demo;
try {
/* Load the "Demo" class from memory */
System.out.println("About to load class Demo");
c = loader.loadClass("Demo", true);
System.out.println("About to instantiate class Demo");
demo = c.newInstance();
System.out.println("Got Demo class loaded: " + demo);
/* Now try to call a method */
Method mi = c.getMethod("test", null);
mi.invoke(demo, null);
} catch (InvocationTargetException e) {
// The invoked method threw an exception. We get it
// wrapped up inside another exception, hence the
// extra call here:
e.getTargetException().printStackTrace();
System.out.println("Could not run test method");
} catch (Exception e) {
e.printStackTrace();
System.out.println("Could not run test method");
}
/** Try to load some arbitrary class, to see if our
* ClassLoader gets called.
*/
System.out.println("Trying to load an unrelated class");
java.awt.image.DirectColorModel jnk = new java.awt.image.DirectColorModel(24,8,8,8);
System.out.println("Load an unrelated class - was your ClassLoader called?");
/** Try to instantiate a second ClassLoader */
System.out.println("Trying to install another ClassLoader");
ClassLoaderDemo1 loader2 = new ClassLoaderDemo1();
System.out.println("Instantiated another ClassLoader...");
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