How can get an instance from .class file in java

问题

Frankly, I do not know even it is possible or not. But what I am trying to do is just like below.

1. I made a class file from ClassFile.java via javac command in terminal.
2. Then I want to get an instance from .java file or .class file.
3. Next, I made another project in eclipse, As you guess this project path and upper file path are completely different. For instance, ClassFile.java/class file can be located in '~/Downloads' folder, the other hand, new eclipse project can be in '~/workspace/'.
4. So I read file which referred in step 1 by FileInputStream.

5. From here, I just paste my code.

   public class Main {

   private static final String CLASS_FILE_PATH = 
           "/Users/juneyoungoh/Downloads/ClassFile.class";
   
   private static final String JAVA_FILE_PATH = 
           "/Users/juneyoungoh/Downloads/ClassFile.java";
   
   private static Class getClassFromFile(File classFile) throws Exception {
       System.out.println("get class from file : [" + classFile.getCanonicalPath() + " ]");
       Object primativeClz = new Object();
       ObjectInputStream ois = null;
       ois = new ObjectInputStream(new FileInputStream(classFile));
       primativeClz = ois.readObject();
       ois.close();
       return primativeClz.getClass();
   }
   
   public static void main(String[] args) throws Exception {
       getClassInfo(getClassFromFile(new File(CLASS_FILE_PATH)));
   }
   

   }

just like your assumption, this code has errors. For example, it shows :

java.io.StreamCurruptedException: invalid stream header : CAFEBABE

this there any way to get object instance from .class file or .java file?

P.S. I wish do not use extra libraries.

回答1:

private static final String CLASS_FOLDER =
        "/Users/juneyoungoh/Downloads/";

private static Class getClassFromFile(String fullClassName) throws Exception {
    URLClassLoader loader = new URLClassLoader(new URL[] {
            new URL("file://" + CLASS_FOLDER)
    });
    return loader.loadClass(fullClassName);
}

public static void main( String[] args ) throws Exception {
    System.out.println((getClassFromFile("ClassFile"));
}

来源：https://stackoverflow.com/questions/21258032/how-can-get-an-instance-from-class-file-in-java