使用 URLClassLoader 加载 .class 文件

Question

I'm aware that this question has been asked before:

How to use URLClassLoader to load a *.class file?

However I don't really understand due to the lack of example. I'm currently working on a project and trying to load user-given .class objects which can be located in any directories on the machine.

//Create URL to hash function class file
URL url_hashFunctionPath = new URL("file:///" + _sHashFunctionFilePath);

//Packet URL to a URL array to be used by URLClassLoader
URL[] urlA_hashFunctionPath = {url_hashFunctionPath};

//Load URL for hash function via a URL class loader
URLClassLoader urlCl_hashFunctionClassLoader = new URLClassLoader(urlA_hashFunctionPath);

//Load user hash function into class to initialize later (TEST: HARD CODE FOR NOW)
m_classUserHashClass = urlCl_hashFunctionClassLoader.loadClass(_sUserHashClassName);

The last line gave me a ClassNotFoundException, from my experiment & understanding the user-given class function has to be in the classpath?

PS: 1st time posting questions feel free to correct me where I did not follow the appropriate manner.

//SOLUTION

The solution that I arrived at with the generous help of [WillShackleford][1], this solution can load the a .class file in a given filepath. For more information refer to code and their given comments.

//The absolute file path to the class that is to be loaded (_sHashFunctionFilePath = absolute file path)
String pathToClassFile = _sHashFunctionFilePath;
System.out.println("File to class: " + _sHashFunctionFilePath);

//Declare the process builder to execute class file at run time (Provided filepath to class)
ProcessBuilder pb = new ProcessBuilder("javap", pathToClassFile);
try
{
    //Start the process builder
    Process p = pb.start();

    //Declare string to hold class name
    String classname = null;
    //Declare buffer reader to read the class file & get class name
    try(BufferedReader br = new BufferedReader(new InputStreamReader(p.getInputStream())))
    {
        String line;
        while(null != (line = br.readLine()))
        {
            if(line.startsWith("public class"))
            {
                classname = line.split(" ")[2];
                break;
            }
        }
        System.out.println("classname = " + classname);
    }
    catch(IOException _error)
    {

    }

    //Declare file path to directory containing class to be loaded
    String pathToPackageBase = pathToClassFile.substring(0, pathToClassFile.length() - (classname + ".class").length());
    System.out.println("pathToPackageBase = " + pathToPackageBase);

    try
    {
        //Create class to hold the class to be loaded via a URL class loader
        Class clss = new URLClassLoader(
                new URL[]{new File(pathToPackageBase).toURI().toURL()}
        ).loadClass(classname);

        //Create ab object/instance of said class
        Object test = clss.newInstance();

        //Declare & create requested method from user hash function class (Method is work & takes no arguments)
        Method method = clss.getMethod("work", null);
        method.invoke(test, null);
    }

Answer 1

In the directory /home/shackle/somedir/classes/pkg I have a file Test.class created from a java file with package pkg; eg :

package pkg;

public class Test {

    public String toString() {
        return "secret_string";
    }
}

Then I load it with :

System.out.println(new URLClassLoader(
        new URL[]{new File("/home/shackle/somedir/classes").toURI().toURL()}
).loadClass("pkg.Test").newInstance().toString());

Notice that I do not put the pkg/Test in the URL string but the load class argument has the pkg. prefix.

You can get the class name directly from the file like this:

Class clsReaderClss = ClassLoader.getSystemClassLoader().loadClass("jdk.internal.org.objectweb.asm.ClassReader");
System.out.println("clsReaderClss = " + clsReaderClss);
Constructor con = clsReaderClss.getConstructor(InputStream.class);
Object reader = con.newInstance(new FileInputStream(directFile));
Method m = clsReaderClss.getMethod("getClassName");
String name = m.invoke(reader).toString().replace('/', '.');
System.out.println("name = " + name);

An alternative that doesn't require access to internal classes.

String pathToClassFile = "/home/shackle/somedir/classes/pkg/Test.class";
ProcessBuilder pb = new ProcessBuilder("javap",pathToClassFile);
Process p = pb.start();
String classname = null;
try(BufferedReader br = new BufferedReader(new InputStreamReader(p.getInputStream()))) {
   String line;
   while(null != (line = br.readLine())) {
       if(line.startsWith("public class")) {
           classname = line.split(" ")[2];
           break;
       }
   }
}
System.out.println("classname = " + classname);

Class can then be loaded with:

String pathToPackageBase = pathToClassFile.substring(0, pathToClassFile.length() - (classname + ".class").length());
System.out.println("pathToPackagBase = " + pathToPackageBase);
Class clss = new URLClassLoader(
        new URL[]{new File(pathToPackageBase).toURI().toURL()}
).loadClass(classname);
System.out.println(clss.newInstance().toString());

Answer 2

Your _sHashFunctionFilePath needs to have the package name of the target class removed from it, so the ClassLoader will look in _sHashFunctionFilePath + package.name + HashFunction.class as the path to the file. If you don't do that, the ClassLoader won't be able to find the file.

So if the target class is my.great.HashFunction in HashFunction.class , then it needs to be in a directory called my/great/ if you want to use URLClassLoader. Then, you'd use /path/to as the file:/// URL for your URLClassLoader if the .class file was actually found in /path/to/my/great/HashFunction.class .