使用 Java 在类路径中添加 .jar 文件

Question

我正在尝试用 Java 开发一个脚本，该脚本在指定目录中查找所有 .jar 文件，然后将它们查找到 classpath 并在某些条件下调用它们的 main() 方法。这是我的 Java 信息​​：

java version "1.6.0_24"
OpenJDK Runtime Environment (IcedTea6 1.11.5) (6b24-1.11.5-0ubuntu1~12.04.1)
OpenJDK Server VM (build 20.0-b12, mixed mode)

这是当前工作目录的ls：

clojure.jar 
loader.class
loader.java 

我正在执行以下操作，以便将 clojure.jar 添加到 classpath 并调用其 main 方法：

import java.io.File;

import java.net.URL;
import java.net.URLClassLoader;
import java.net.MalformedURLException;

import java.lang.reflect.Method;

public final class loader {

  public static void main (String[] args) {
    try {
      printClasspathString();
      System.out.println ("**********");
      URL[] classesRepo = { new File("clojure.jar").toURI ().toURL (),
                            new File(System.getProperty("user.dir")).toURI ().toURL ()};
      ClassLoader currentThreadClassLoader = Thread.currentThread().getContextClassLoader();
      URLClassLoader urlClassLoader = new URLClassLoader( classesRepo, currentThreadClassLoader);
      Thread.currentThread().setContextClassLoader(urlClassLoader);
      printClasspathString();
    } catch (Exception ex) {
      System.out.println(ex.getMessage ());
    }

    //Do I miss something here?

    String mainClassName="clojure.main";

    Class<?> mainClass = null;
    try {
      mainClass = Class.forName(mainClassName);
    }
    catch (Exception ex) {
      throw new IllegalArgumentException("class not found in your jar file " + mainClassName);
    }

    Method mainMethod = null;
    try {
      mainMethod = mainClass.getMethod("main", String[].class);
    }
    catch (Exception ex) {
      throw new IllegalArgumentException("class to launch (" + mainClassName + ") does not have a public static void main(String[]) method");
    }

    try {
      mainMethod.invoke(null, (Object) args);
    } catch (Exception ex) {
      System.out.println(ex.getMessage ());
    }

  }

  public static void printClasspathString() {
    ClassLoader applicationClassLoader = Thread.currentThread().getContextClassLoader();
    if (applicationClassLoader == null) {
      applicationClassLoader = ClassLoader.getSystemClassLoader();
    }
    URL[] urls = ((URLClassLoader)applicationClassLoader).getURLs();
    for(int i=0; i < urls.length; i++) {
      System.out.println (urls[i].getFile());
    }
  }

}

不幸的是，加载器没有按预期工作：

$ java -cp . loader
/home/proofit404/data/downloads/clojure-loader/
**********
/home/proofit404/data/downloads/clojure-loader/clojure.jar
/home/proofit404/data/downloads/clojure-loader/
Exception in thread "main" java.lang.IllegalArgumentException: class not found in your jar file clojure.main
    at loader.main(loader.java:37)

不过，如果我使用 -cp 选项，一切正常：

$ java -cp .:clojure.jar loader
/home/proofit404/data/downloads/clojure-loader/
/home/proofit404/data/downloads/clojure-loader/clojure.jar
**********
/home/proofit404/data/downloads/clojure-loader/clojure.jar
/home/proofit404/data/downloads/clojure-loader/
Clojure 1.4.0
user=> (System/exit 0)

那么 - 我需要对我的代码进行哪些更改才能使其按预期工作？

Answer 1

我认为问题在于Class.forName(String)方法没有使用线程contextclassloader，而是当前类的classloader：

public static Class<?> forName(String className)
                    throws ClassNotFoundException

Returns the Class object associated with the class or interface with the given string name. Invoking this method is equivalent to:

    Class.forName(className, true, currentLoader)

where currentLoader denotes the defining class loader of the current class.

这意味着您的 URLClassLoader 不会被使用。尝试使用 Class.forName(String,boolean, ClassLoader) 显式传递类加载器：

mainClass = Class.forName(mainClassName, true, urlClassLoader);

Answer 2

试试这个代码并遵循下面给出的 cmets：

import java.net.URL;
import java.io.IOException;
import java.net.URLClassLoader;
import java.net.MalformedURLException;

public class JarLoader extends URLClassLoader {
    public JarLoader(URL[] urls) {
        super(urls);
    }

    public void addFile(String path) throws MalformedURLException {
        String urlPath = "jar:file://" + path + "!/";
        addURL(new URL(urlPath));
    }

    public static void main(String args[]) {
        try {
            System.out.println("First attempt...");
            Class.forName("org.gjt.mm.mysql.Driver");
            //specify your class name above
        } catch (Exception ex) {
            System.out.println("Failed.");
        }

        try {
            URL urls[] = {};

            JarLoader cl = new JarLoader(urls);
            cl
                    .addFile("/opt/mysql-connector-java-5.0.4/mysql-connector-java-5.0.4-bin.jar");
            // give your jar file above.
            System.out.println("Second attempt...");
            cl.loadClass("org.gjt.mm.mysql.Driver");
            //specify your class name above
            System.out.println("Success!");
        } catch (Exception ex) {
            System.out.println("Failed.");
            ex.printStackTrace();
        }
    }
}