通过ClassLoader调用外部jar包

我们大家都知道，每个运行中的线程都有一个成员contextClassLoader，用来在运行时动态地载入其它类。

系统默认的contextClassLoader是systemClassLoader，所以一般而言java程序在执行时可以使用JVM自带的类、$JAVA_HOME/jre/lib/ext/中的类和$CLASSPATH/中的类，对于非默认的jar，一般只能手动在配置环境添加。

但事实上，我们可以通过Thread.currentThread().setContextClassLoader()更改当前线程的contextClassLoader行为，实现在程序内加载外部jar。

PS:
ClassLoader的工作原理是：
1) 线程需要用到某个类时，contextClassLoader将被请求来载入该类
2) contextClassLoader请求它的父ClassLoader来完成该载入请求
3) 如果父ClassLoader无法载入类，则contextClassLoader试图自己来载入


package org.loon.framework.jar;

/** *//**
 * <p>Title: LoonFramework</p>
 * <p>Description:JarLoader，用于jar包的外部操作</p>
 * <p>Copyright: Copyright (c) 2007</p>
 * <p>Company: LoonFramework</p>
 * @author chenpeng  
 * @email：[email]ceponline@yahoo.com.cn[/email] 
 * @version 0.1
 */
import java.io.BufferedInputStream;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.lang.reflect.Method;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Hashtable;
import java.util.jar.JarEntry;
import java.util.jar.JarInputStream;
import java.util.jar.Manifest;

public class JarLoader extends ClassLoader ...{
    //资源缓存
    public static Hashtable resources = new Hashtable();
 
    public static JarLoader loader = new JarLoader();

    public static Class load(byte[] resource) throws Exception ...{
        // 主函数所在类全称
        String mainClassName = "";
        //class资源及实体缓存
        ArrayList classNames = new ArrayList();
        ArrayList classBuffers = new ArrayList();
        // 存储依赖类
        HashMap depends = new HashMap();
        // 将byte[]转为JarInputStream
        JarInputStream jar = new JarInputStream(new ByteArrayInputStream(
                resource));
        Manifest manifest = jar.getManifest();
        // 当Main-Class被声明时,获得主函数所在类全称
        if (manifest != null) ...{
            mainClassName = manifest.getMainAttributes().getValue("Main-Class");
        }
        // 依次获得对应JAR文件中封装的各个被压缩文件的JarEntry
        JarEntry entry;
        while ((entry = jar.getNextJarEntry()) != null) ...{
            // 当找到的entry为class时
            if (entry.getName().toLowerCase().endsWith(".class")) ...{
                // 将类路径转变为类全称
                String name = entry.getName().substring(0,
                        entry.getName().length() - ".class".length()).replace(
                        '/', '.');
                // 加载该类
                byte[] data = getResourceData(jar);
                // 缓存类名及数据
                classNames.add(name);
                classBuffers.add(data);

            } else ...{
                // 非class结尾但开头字符为'/'时
                if (entry.getName().charAt(0) == '/') ...{
                    resources.put(entry.getName(), getResourceData(jar));
                // 否则追加'/'后缓存    
                } else ...{
                    resources.put("/" + entry.getName(), getResourceData(jar));
                }
            }
        }
        //当获得的main-class名不为空时
        while (classNames.size() > 0) ...{
            //获得类路径全长
            int n = classNames.size();
            for (int i = classNames.size() - 1; i >= 0; i--) ...{
                try ...{
                    //查询指定类
                    loader.defineClass((String) classNames.get(i),
                            (byte[]) classBuffers.get(i), 0,
                            ((byte[]) classBuffers.get(i)).length);
                    //获得类名
                    String pkName = (String) classNames.get(i);
                    if (pkName.lastIndexOf('.') >= 0) ...{
                        pkName = pkName
                                .substring(0, pkName.lastIndexOf('.'));
                        if (loader.getPackage(pkName) == null) ...{
                            loader.definePackage(pkName, null, null, null,
                                    null, null, null, null);
                        }
                    }
                    //查询后删除缓冲
                    classNames.remove(i);
                    classBuffers.remove(i);
                } catch (NoClassDefFoundError e) ...{
                    depends.put((String) classNames.get(i), e.getMessage()
                            .replaceAll("/", "."));
                } catch (UnsupportedClassVersionError e) ...{
                    //jre版本错误提示
                    throw new UnsupportedClassVersionError(classNames.get(i)
                            + ", " + System.getProperty("java.vm.name") + " "
                            + System.getProperty("java.vm.version") + ")");
                }
            }
            if (n == classNames.size()) ...{
                for (int i = 0; i < classNames.size(); i++) ...{
                    System.err.println("NoClassDefFoundError:"
                            + classNames.get(i));
                    String className = (String) classNames.get(i);
                    while (depends.containsKey(className)) ...{
                        className = (String) depends.get(className);
                    }
                }
                break;
            }
        }
        try ...{
            //加载
            Thread.currentThread().setContextClassLoader(loader);
            // 获得指定类,查找其他类方式相仿
            return Class.forName(mainClassName, true, loader);
        } catch (ClassNotFoundException e) ...{
            String className = mainClassName;
            while (depends.containsKey(className)) ...{
                className = (String) depends.get(className);
            }
            throw new ClassNotFoundException(className);
        }
    }

    /** *//**
     * 获得指定路径文件的byte[]形式
     * @param name
     * @return
     */
    final static public byte[] getDataSource(String name) ...{
        FileInputStream fileInput;
        try ...{
            fileInput = new FileInputStream(new File(name));
        } catch (FileNotFoundException e) ...{
            fileInput = null;
        }
        BufferedInputStream bufferedInput = new BufferedInputStream(fileInput);
        return getDataSource(bufferedInput);
    }
    
    /** *//**
     * 获得指定InputStream的byte[]形式
     * @param name
     * @return
     */
    final static public byte[] getDataSource(InputStream is) ...{
        if (is == null) ...{
            return null;
        }
        ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
        byte[] arrayByte = null;
        try ...{
            byte[] bytes = new byte[8192];
            bytes = new byte[is.available()];
            int read;
            while ((read = is.read(bytes)) >= 0) ...{
                byteArrayOutputStream.write(bytes, 0, read);
            }
            arrayByte = byteArrayOutputStream.toByteArray();
        } catch (IOException e) ...{
            return null;
        } finally ...{
            try ...{
                if (byteArrayOutputStream != null) ...{
                    byteArrayOutputStream.close();
                    byteArrayOutputStream = null;
                }
                if (is != null) ...{
                    is.close();
                    is = null;
                }

            } catch (IOException e) ...{
            }
        }
        return arrayByte;
    }

    /** *//**
     * 获得指定JarInputStream的byte[]形式
     * @param jar
     * @return
     * @throws IOException
     */
     final static private byte[] getResourceData(JarInputStream jar)
            throws IOException ...{
        ByteArrayOutputStream data = new ByteArrayOutputStream();
        byte[] buffer = new byte[8192];
        int size;
        while (jar.available() > 0) ...{
            size = jar.read(buffer);
            if (size > 0) ...{
                data.write(buffer, 0, size);
            }
        }
        return data.toByteArray();
    }

     /** *//**
      * 重载的getResource,检查是否重复包含
      */
    public URL getResource(String name) ...{
        if (resources.containsKey("/" + name)) ...{
            try ...{
                return new URL("file:///" + name);
            } catch (MalformedURLException e) ...{
                e.printStackTrace();
            }
        }
        return super.getResource(name);
    }

    /** *//**
     * 执行指定class类
     * @param clz
     * @param methodName
     * @param args
     */
    public static void callVoidMethod(Class clz, String methodName,
            String[] args) ...{
        Class[] arg = new Class[1];
        arg[0] = args.getClass();
        try ...{
            Method method = clz.getMethod(methodName, arg);
            Object[] inArg = new Object[1];
            inArg[0] = args;
            method.invoke(clz, inArg);
        } catch (Exception e) ...{
            System.err.println(e.getMessage());
        }
    }
    
     /** *//**
      * 重载的getResourceAsStream,检查是否重复包含
      */
    public InputStream getResourceAsStream(String name) ...{
        if (name.charAt(0) == '/') ...{
            name = name.substring(1);
        }
        if (resources.containsKey("/" + name)) ...{
            return new ByteArrayInputStream((byte[]) resources.get("/" + name));
        }
        return super.getResourceAsStream(name);
    }

}


运行示例：

package org.loon.framework.jar;


/** *//**
 * <p>
 * Title: LoonFramework
 * </p>
 * <p>
 * Description:从外部启动jar包
 * </p>
 * <p>
 * Copyright: Copyright (c) 2007
 * </p>
 * <p>
 * Company: LoonFramework
 * </p>
 * 
 * @author chenpeng
 * @email：[email]ceponline@yahoo.com.cn[/email]
 * @version 0.1
 */
public class JarTest ...{

    public static void main(String[] args) ...{
        //将jar包转为byte[]
        byte[] resource = JarLoader.getDataSource("D:/fps_test.jar");
        try ...{
            //通过byte[]获得主函数所在类
            Class clz = JarLoader.load(resource);
            //调用main函数
            JarLoader.callVoidMethod(clz, "main", new String[] ...{""});
        } catch (Exception e) ...{
            e.getStackTrace();
        }

    }

}

这时即使指定jar包没有被我们添加到lib中，程序依旧被顺利启动了。

但是有个缺点，在没有优化的前提下，这种直接加载外部包的速度在jvm会有很大损耗。

我们可以看到，fps值降低到正常值的50%左右（此fps实例见我CSDN以前的文章），所以并不适合直接运用在需要复杂运算的jar中类调用上（当然，有兴趣的话，可以在代码中优化，我自己在项目中写的另一个例子速度明显比这个快）。但是对于运算量小的外部jar调用，还是很方便的。 

本文转自 cping 51CTO博客，原文链接:http://blog.51cto.com/cping1982/130181