我正在编写一个游戏引擎，我需要在其中分析程序中提到的每一个类。由于这是一个游戏引擎，它会以 JAR 文件的形式附加到客户的项目中。从该 JAR 文件中，我需要能够扫描客户端正在使用的每个类。
所以我想我应该创建一个自定义的 ClassLoader！通过重写 ClassLoader，我可以查看加载时正在使用的每个类。
我开始玩一些 ClassLoaders。这是我所做的：（我从 JavaWorld 偷了这个类加载器只是为了玩它）
public class SimpleClassLoader extends ClassLoader {
private HashMap<String, Class> classes = new HashMap<String, Class>();
public SimpleClassLoader() {
}
/**
* This sample function for reading class implementations reads
* them from the local file system
*/
private byte getClassImplFromDataBase(String className)[] {
System.out.println(" >>>>>> Fetching the implementation of "+className);
byte result[];
try {
FileInputStream fi = new FileInputStream("store\\"+className+".impl");
result = new byte[fi.available()];
fi.read(result);
return result;
} catch (Exception e) {
/*
* If we caught an exception, either the class wasnt found or it
* was unreadable by our process.
*/
return null;
}
}
/**
* This is a simple version for external clients since they
* will always want the class resolved before it is returned
* to them.
*/
public Class loadClass(String className) throws ClassNotFoundException {
return (loadClass(className, true));
}
/**
* This is the required version of loadClass which is called
* both from loadClass above and from the internal function
* FindClassFromClass.
*/
public synchronized Class loadClass(String className, boolean resolveIt)
throws ClassNotFoundException {
Class result;
byte classData[];
System.out.println(className);
System.out.println(" >>>>>> Load class : "+className);
/* Check our local cache of classes */
result = (Class)classes.get(className);
if (result != null) {
System.out.println(" >>>>>> returning cached result.");
return result;
}
/* Check with the primordial class loader */
try {
result = super.findSystemClass(className);
System.out.println(" >>>>>> returning system class (in CLASSPATH).");
return result;
} catch (ClassNotFoundException e) {
System.out.println(" >>>>>> Not a system class.");
}
/* Try to load it from our repository */
classData = getClassImplFromDataBase(className);
if (classData == null) {
throw new ClassNotFoundException();
}
/* Define it (parse the class file) */
result = defineClass(classData, 0, classData.length);
if (result == null) {
throw new ClassFormatError();
}
if (resolveIt) {
resolveClass(result);
}
classes.put(className, result);
System.out.println(" >>>>>> Returning newly loaded class.");
return result;
}
}
然后我决定测试它：
public class Test
{
public static void main(String args[]) throws ClassNotFoundException
{
SimpleClassLoader s = new SimpleClassLoader();
Thread.currentThread().setContextClassLoader(s);
Foo myfoo = new Foo(); //local class
ArrayList myList = new ArrayList(); //class from JAR file
//both should have been loaded from my SimpleClassLoader
System.out.println(s + "\n\tshould be equal to\n" + myfoo.getClass().getClassLoader());
System.out.println("\tand also to: \n" + myList.getClass().getClassLoader());
/*
OUTPUT:
SimpleClassLoader@57fee6fc
should be equal to
sun.misc.Launcher$AppClassLoader@51f12c4e
and also to:
null
***
bizarre results: why are ArrayList and Foo not being loaded by my classloader?
*/
}
}
创建自定义 ClassLoader 是否是解决顶部描述的问题的正确方法？
为什么要调用系统类加载器？如何强制 JVM对所有线程使用我的类加载器？（不仅仅是第一个线程，我希望创建的每个新线程都能自动使用我的类加载器）
据我了解，通过“委托系统”，父类加载器首先尝试加载类；这可能是我的 ClassLoader 没有加载任何内容的原因。如果我是正确的，如何禁用此功能？如何让我的类加载器进行加载？