método loadClass ()
, Garantía del modelo de delegación principal, destruir el modelo de delegación principal reescribir método loadClass
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
synchronized (getClassLoadingLock(name)) {
// First, check if the class has already been loaded
//检查是否被加载过
Class<?> c = findLoadedClass(name);
if (c == null) {
long t0 = System.nanoTime();
try {
//如果父加载器不是空,调用父加载器的loadclass
if (parent != null) {
c = parent.loadClass(name, false);
} else {
//父加载器为空,查询是否被根加载器加载过,没有返回空,抛出异常
c = findBootstrapClassOrNull(name);
}
} catch (ClassNotFoundException e) {
// ClassNotFoundException thrown if class not found
// from the non-null parent class loader
//如果父加载器没找到,抛出ClassNotFound异常
}
//如果父加载器找不到,调用本身的findClass方法加载
if (c == null) {
// If still not found, then invoke findClass in order
// to find the class.
long t1 = System.nanoTime();
c = findClass(name);
// this is the defining class loader; record the stats
sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
sun.misc.PerfCounter.getFindClasses().increment();
}
}
if (resolve) {
resolveClass(c);
}
return c;
}
}
Método findclass de URLClassLoader
Custom ClassLoader, anula el método findClass
/**
* Finds and loads the class with the specified name from the URL search
* path. Any URLs referring to JAR files are loaded and opened as needed
* until the class is found.
*
* @param name the name of the class
* @return the resulting class
* @exception ClassNotFoundException if the class could not be found,
* or if the loader is closed.
* @exception NullPointerException if {@code name} is {@code null}.
* 在指定的URL路径寻找class
*/
protected Class<?> findClass(final String name)
throws ClassNotFoundException
{
final Class<?> result;
try {
result = AccessController.doPrivileged(
new PrivilegedExceptionAction<Class<?>>() {
public Class<?> run() throws ClassNotFoundException {
String path = name.replace('.', '/').concat(".class");
Resource res = ucp.getResource(path, false);
if (res != null) {
try {
return defineClass(name, res);
} catch (IOException e) {
throw new ClassNotFoundException(name, e);
}
} else {
return null;
}
}
}, acc);
} catch (java.security.PrivilegedActionException pae) {
throw (ClassNotFoundException) pae.getException();
}
if (result == null) {
throw new ClassNotFoundException(name);
}
return result;
}
ClassLoader personalizado
public class MyClassLoader extends ClassLoader {
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
File f = new File("c:/test/", name.replace(".", "/").concat(".class"));
FileInputStream fis = null;
ByteArrayOutputStream baos = null;
try {
fis = new FileInputStream(f);
baos = new ByteArrayOutputStream();
int b = 0;
while ((b = fis.read()) != 0) {
baos.write(b);
}
byte[] bytes = baos.toByteArray();
//defineClass 根据字节数组定义Class,跟进去时native方法
return defineClass(name, bytes, 0, bytes.length);
} catch (Exception e) {
e.printStackTrace();
} finally {
if(baos!=null){
try {
baos.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if(fis!=null){
try {
fis.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
return super.findClass(name); //throws ClassNotFoundException
}