[英]How to reload a loaded class with different ClassLoader in Java?
[英]Reload class which is loaded using custom classloader
以下是我的自定义类加载器类。 我使用以下javaargs将其设置为默认类加载器。
-Djava.system.class.loader=MyCustomClassLoader
import java.io.*;
public class MyCustomClassLoader extends ClassLoader {
public MyCustomClassLoader() { super(); }
public MyCustomClassLoader(ClassLoader parent) { super(parent); }
protected Class loadClass(String name, boolean resolve ) throws ClassNotFoundException {
Class c = super.loadClass(name, resolve );
System.out.println( "name: " + name + ", class: " + c);
return c;
}
}
此时,当我启动服务器时,所有类都使用上面的自定义类加载器类加载。 我希望根据需要更新/重新加载属于特定程序包(例如com.test)的类的类定义。 我该怎么做?
您应该创建一个类加载器,该类加载器将自己加载custor类,不要让它们超级加载。 试试我的例子,可能有助于理解
package test;
import java.io.IOException;
import java.net.MalformedURLException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.HashMap;
import java.util.Map;
class MyClassLoader extends ClassLoader {
private Map<String, Class<?>> loadedClasses = new HashMap<>();
private String dir;
public MyClassLoader(String dir) throws MalformedURLException {
this.dir = dir;
}
protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
Class<?> c = findLoadedClass(name);
if (c != null) {
return c;
}
c = loadedClasses.get(name);
if (c != null) {
return c;
}
try {
String path = dir + name.replace(".","/") + ".class";
byte[] bytes = Files.readAllBytes(Paths.get(path));
c = defineClass(name, bytes, 0, bytes.length, null);
if (resolve) {
resolveClass(c);
}
return c;
} catch (IOException ex) {
return super.loadClass(name, resolve);
}
}
}
public class ChildFirstClassLoader {
public static void main(String[] args) throws Exception {
ClassLoader cl1 = new MyClassLoader("target/classes/");
Object x1 = cl1.loadClass("test.X1").newInstance();
System.out.println(x1.getClass().getClassLoader());
cl1 = null;
ClassLoader cl2 = new MyClassLoader("target/classes/");
x1 = cl2.loadClass("test.X1").newInstance();
System.out.println(x1.getClass().getClassLoader());
}
}
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