出于某种原因,尝试将一个类 X
转换为另一个类 Y
,在第三个类 Z
中抛出ClassCastException
。这对我来说似乎是错误的,因为类 X
扩展了另一个类 Y
。是否有任何特定原因导致类 X
无法转换为 Y
,即使 X
对其进行了扩展?
引用如下代码:
是
:
public abstract class Y {
/**
* Called when the extension is enabled.
*/
public void onEnable() {
}
}
X
:
public class X extends Y {
@Override
public void onEnable() {
// Extension specific code.
}
}
Z
:(此代码是 ClassCastException
的来源代码。)
public class Z {
private boolean loadExtension(ExtensionDescription description) {
try {
URLClassLoader loader = new ExtensionClassLoader(new URL[]{description.getFile().toURI().toURL()});
Y y = (Y) loader.loadClass(description.getMain()).newInstance();
} catch (Throwable t) {}
}
}
如果已知 loader.loadClass(description.getMain()).newInstance();
会创建 X
的新实例,那么为什么要转换为 Y
导致 ClassCastException
?
最佳答案
为了进一步说明,这里有一个例子:
创建自定义 ClassLoader
,例如下面(从 here 复制)
package com.dd;
import java.io.BufferedInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;
public class CustomClassLoader extends ClassLoader {
/**
* The HashMap where the classes will be cached
*/
private Map<String, Class<?>> classes = new HashMap<String, Class<?>>();
@Override
public String toString() {
return CustomClassLoader.class.getName();
}
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
if (classes.containsKey(name)) {
return classes.get(name);
}
byte[] classData;
try {
classData = loadClassData(name);
} catch (IOException e) {
throw new ClassNotFoundException("Class [" + name
+ "] could not be found", e);
}
Class<?> c = defineClass(name, classData, 0, classData.length);
resolveClass(c);
classes.put(name, c);
return c;
}
/**
* Load the class file into byte array
*
* @param name
* The name of the class e.g. com.codeslices.test.TestClass}
* @return The class file as byte array
* @throws IOException
*/
private byte[] loadClassData(String name) throws IOException {
BufferedInputStream in = new BufferedInputStream(
ClassLoader.getSystemResourceAsStream(name.replace(".", "/")
+ ".class"));
ByteArrayOutputStream out = new ByteArrayOutputStream();
int i;
while ((i = in.read()) != -1) {
out.write(i);
}
in.close();
byte[] classData = out.toByteArray();
out.close();
return classData;
}
}
这里是类 Z
package com.dd;
import java.lang.reflect.InvocationTargetException;
public class Z {
public static void main(String[] args) throws ClassNotFoundException,
InstantiationException, IllegalAccessException,
NoSuchMethodException, SecurityException, IllegalArgumentException,
InvocationTargetException {
CustomClassLoader loader = new CustomClassLoader();
Class<?> c1 = loader.findClass("com.dd.X");
System.out.println("Classloader:: "+ X.class.getClassLoader());
System.out.println("Classloader:: "+ loader.findClass("com.dd.X").getClassLoader());
X x = (X)c1.newInstance();
}
}
这是输出:
Classloader:: sun.misc.Launcher$AppClassLoader@781fb069
Classloader:: com.dd.CustomClassLoader
Exception in thread "main" java.lang.ClassCastException: com.dd.X cannot be cast to com.dd.X
at com.dd.Z.main(Z.java:18)
关于java - 无法将类 "X"转换为类 "Y",即使 X 扩展了 Y?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/31623155/