自定义classLoader加载hibernate项目下的class文件并执行其中的代码

1 加载某一根目录下的class文件（此处代码转载）

public class TestClassLoader extends ClassLoader{
	
	private static final int CAPACITY = 1024;
	
	private String baseClassName;//根目录路径，例如某一个工程的classes目录
	
	public TestClassLoader(String baseClassName) {
		if (!baseClassName.endsWith(File.separator)) {
			this.baseClassName = baseClassName + File.separator;
		} else {
			this.baseClassName = baseClassName;
		}
	}

	protected Class<?> findClass(final String name) throws ClassNotFoundException {
		byte[] b = this.getByteFromClass(name);
		return this.defineClass(name, b, 0, b.length);
	}
	
	private byte[] getByteFromClass(final String name) {
		String path = StringUtils.replace(name, ".", File.separator) + ".class";
		File f = new File(baseClassName + path);
		if (!f.exists() || f.isDirectory()) {
			throw new RuntimeException(name + "is not exists!");
		}
		FileInputStream fileInput = null;
		
		ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
		byte[] b = new byte[CAPACITY], classByte = null;
		try {
			int readLen;
			fileInput = new FileInputStream(f);
			
			while ((readLen = fileInput.read(b)) != -1) {
				byteArrayOutputStream.write(b, 0, readLen);
			}
			
			classByte = byteArrayOutputStream.toByteArray();
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		} finally {
			try {
				if (fileInput != null) {
					fileInput.close();
				}
			} catch (IOException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} finally {
				fileInput = null;
			}
			
			try {
				if (byteArrayOutputStream != null) {
					byteArrayOutputStream.close();
				}
			} catch (IOException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} finally {
				byteArrayOutputStream = null;
			}
			
		}
		return classByte;
	}
	
	public static void main(String[] args) throws ClassNotFoundException {
		TestClassLoader loader = new TestClassLoader("H:/eclipse/workspace/hib/target/classes");
		Class employerManager = loader.loadClass("employ.Employer");
		System.out.println(employerManager.getName());
		Method[] methods = employerManager.getDeclaredMethods();
		for (Method method : methods) {
			System.out.println(method.getName());
		}
	}
}

2 利用URLClassLoader来加载jar包，主要代码如下：
 

URLClassLoader systemClassLoader = (URLClassLoader) ClassLoader.getSystemClassLoader();
		   Method addURL = URLClassLoader.class.getDeclaredMethod("addURL",new Class[] { URL.class });
		   addURL.setAccessible(true);
		   
		   //add all the jar file path to classpath	   
		   for (File jarFile : jarFiles)
		   {
			   addURL.invoke(systemClassLoader, new Object[] { jarFile.toURL() });
		   } 

这部分代码主要用在服务器启动时加载一些jar包
3 自定义ClassLoader来加载jar包
  

public class MyClassLoader extends URLClassLoader {
	//加载类
	protected Class<?> findClass(String name) throws ClassNotFoundException {
		return super.findClass(name);
	}


	public MyClassLoader(URL[] urls) {
		super(urls);
	}
	
	

	public MyClassLoader(URL[] urls, ClassLoader parent,
			URLStreamHandlerFactory factory) {
		super(urls, parent, factory);
		// TODO Auto-generated constructor stub
	}



	public MyClassLoader(URL[] urls, ClassLoader parent) {
		super(urls, parent);
		// TODO Auto-generated constructor stub
	}
	
	protected void addURL(URL url) {
		super.addURL(url);
	}
	
         //查找各种配置文件，例如hibernate.cfg.xml或者hibernate项目下各对象的.hbm.xml文件
	public URL findResource(final String name) {
		URL url = null;
		url = super.findResource(name);
		URL contextUrl = this.getResource(".");
                  //获取该类的classpath
		String contextPath = contextUrl.getPath();
		try {
			if (url == null) {
				if (name.startsWith(File.separator)) {
					String actualName = name.substring(1);
					url = new URL("file://" + contextPath + actualName);
				} else {
					url = new URL("file://" + contextPath + name);
				}
			}
		} catch (MalformedURLException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		return url;
    }

}

  将该类可以放在在一java项目中。
   现在我们新建一hibernate项目，在该hibernate项目中，有一个EmployerManager类，该类功能实现是对员工进行管理，该类有一个getEmployer方法，其中类EmployerManager的getEmployer方法代码如下：

public List<Employer> getEmployer() {
		Session session = HibernateUtil.getSessionFactory().getCurrentSession();
		session.beginTransaction();
		Criteria criteria = session.createCriteria(Employer.class);
		//criteria.add(Restrictions.le("id", 200L));
		criteria.setFirstResult(10);
		criteria.setMaxResults(50);
		List<Employer> employerList = (List<Employer>)criteria.list();
		for (Employer employer : employerList) {
			System.out.println(employer);
		}
		session.getTransaction().commit();
		return employerList;
	}

我们可以利用上述的MyClassLoader来加载hibernate工程下的classes目录下的jar包、classes文件以及xml配置文件
测试类如下（该测试类也在java项目中）：

public class Test {
	
	public static void main(String[] args) throws ClassNotFoundException, IOException, IllegalArgumentException, IllegalAccessException, InvocationTargetException, InstantiationException, SecurityException, NoSuchMethodException {
		File f = new File("H:/eclipse/workspace/hib/target/classes/lib");
		String[] paths = f.list();
		//hib为hibernate工程名，该工程编译后的所有类文件都在H:/eclipse/workspace/hib/target/classes/目录下，同时将该项目用到的jar包放入H:/eclipse/workspace/hib/target/classes/lib目录下
		URL[] urls = {new URL("file:///H:/eclipse/workspace/hib/target/classes/")};
		MyClassLoader loader = new MyClassLoader(urls);
		for (String s : paths) {
			loader.addURL(new URL("file:///H:/eclipse/workspace/hib/target/classes/lib/" + s));
		}
				
		Class employerManager = loader.loadClass("employ.EmployerManager");
		
		Method[] methods = employerManager.getDeclaredMethods();
		for (Method method : methods) {
			if (method.getName().equals("getEmployer")) {
			
				method.invoke(employerManager.newInstance(), null);
			}
			
		}
		
	}
}

我们可以看到在Test类中执行该方法时会正确执行getEmployer的方法
4 总结：
  1  可以得到jar包的加载方法，这里对于采用获取jar包中每一个类文件的字节进行加载的方法不予介绍（javaeye上可以搜到）。
   2  对ClassLoader、URLClassLoader有了一定的了解。对如何加载类、加载jar包、加载资源文件有所了解。对于加载类，我们可以实现findClass方法，对于加载xml，我们可以实现findResource方法
   3  疑问：如果该java项目中的lib包中已经有另一hibernate中的某些包（例如hibernate-3.2.6.ga.jar），且这些包的版本相同或者不相同时结果会如何？冲突？
   4  对于类加载的安全性检查还没有实现。