我尝试了解 java 类加载。我写了例子:
class XLoader extends ClassLoader {
// карта отображения имен классов на файлы .class, где хранятся их определения
HashMap<String, String> mappings;
XLoader(HashMap mappings) {
this.mappings = mappings;
}
public synchronized Class loadClass(String name) throws ClassNotFoundException {
try {
// важно!
// приоритет отдан именно загрузке с помощью встроенного загрузчика
if (!mappings.containsKey(name)) {
System.out.println("loadClass (" + name + ") with parent classloader");
return super.findSystemClass(name);
}
System.out.println("loadClass (" + name + ") with XLoader");
String fileName = mappings.get(name);
FileInputStream fin = new FileInputStream(fileName);
byte[] bbuf = new byte[(int) (new File(fileName).length())];
fin.read(bbuf);
fin.close();
return defineClass(name, bbuf, 0, bbuf.length);
} catch (IOException e) {
e.printStackTrace();
throw new ClassNotFoundException(e.getMessage(), e);
}
}
}
我还有接口(interface):
public interface ISexyInterface {
public void makeBar ();
}
和类:
public class SexyClassForLoader implements ISexyInterface {
...
}
主要方法:
public static void main(String[] args) throws Exception {
HashMap<String, String> mappings = new HashMap();
mappings.put("sur.che.SexyClassForLoader", "D:\\java_things\\custom-classloader\\out\\production\\custom-classloader1\\sur\\che\\SexyClassForLoader.class");
// if comment this line you will see
//mappings.put("sur.che.ISexyInterface", "D:\\java_things\\custom-classloader\\out\\production\\custom-classloader1\\sur\\che\\ISexyInterface.class");
XLoader xloa = new XLoader(mappings);
Class sexy_cla = xloa.loadClass("sur.che.SexyClassForLoader");
System.out.println("class was loaded with " + sexy_cla.getClassLoader());
Object sexy_ob = sexy_cla.newInstance();
System.out.println(sexy_ob.getClass().getClassLoader());
System.out.println(ISexyInterface.class.getClassLoader());
Thread.sleep(100);
ISexyInterface local_sexy = (ISexyInterface) sexy_ob;
local_sexy.makeBar();
}
此示例运行良好并产生以下输出:
loadClass (sur.che.SexyClassForLoader) with XLoader
loadClass (sur.che.ISexyInterface) with parent classloader
loadClass (java.lang.Object) with parent classloader
class was loaded with sur.che.XLoader@7f31245a
loadClass (java.lang.System) with parent classloader
loadClass (java.io.PrintStream) with parent classloader
SexyClassForLoader$$static
SexyClassForLoader$$init
sur.che.XLoader@7f31245a
sun.misc.Launcher$AppClassLoader@232204a1
make bar
但是让我们一起玩:
//mappings.put("sur.che.ISexyInterface", "D:\\java_things\\custom-classloader\\out\\production\\custom-classloader1\\sur\\che\\ISexyInterface.class");
如果取消注释这一行,我会看到以下输出:
loadClass (sur.che.SexyClassForLoader) with XLoader
loadClass (sur.che.ISexyInterface) with XLoader
loadClass (java.lang.Object) with parent classloader
class was loaded with sur.che.XLoader@7f31245a
loadClass (java.lang.System) with parent classloader
loadClass (java.io.PrintStream) with parent classloader
SexyClassForLoader$$static
SexyClassForLoader$$init
sur.che.XLoader@7f31245a
sun.misc.Launcher$AppClassLoader@232204a1
Exception in thread "main" java.lang.ClassCastException: sur.che.SexyClassForLoader cannot be cast to sur.che.ISexyInterface
at sur.che.Loader.main(Loader.java:25)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
请解释这种行为。
附言
我知道如果同一个类加载了 2 个不同的加载器。它算作 2 个不同的类。
最佳答案
If uncomment this line I see following output:......
Class sexy_cla = xloa.loadClass("sur.che.SexyClassForLoader");
sur.che.SexyClassForLoader 由 Xloader 加载,因为它有一个接口(interface) sur.che.ISexyInterface 然后是 jvm 将使用 sur.che.SexyClassForLoader 的 classloader 加载此接口(interface)。
但是对于这个声明:
System.out.println(ISexyInterface.class.getClassLoader());
因为包含main方法的类被AppLoader加载,所以jvm会使用AppLoader加载< em>ISexyInterface.class.
ISexyInterface local_sexy = (ISexyInterface) sexy_ob;
很明显,这个ISexyInterface的类加载器是AppLoader,但是SexyClassForLoader的内部依赖(ISexyInterface)是由加载的xLoader。换句话说,jvm 中有两个 ISexyInterface.class,一个由Xloader 加载,另一个由App 。 sexy_ob 实现了由 Xloader 加载的 ISexyInterface.class。您尝试将 sexy_ob 转换为 ISexyInterface 类型(由 App 加载)。
所以 castException。
如果注释,只有一个 ISexyInterface .class 由 App 加载。所以一切正常。
关于java - 当我使用自定义类加载器时出现 ClassCastException,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/41610912/