自定義ClassLoader動态加載Class

1、自定義ClassLoader代碼如下：

package zmx.classloader.test;

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.util.HashSet;

/*
 *  實作熱部署，自定義ClassLoader，加載的是.class
 */
public class CustomClassLoader extends ClassLoader {

	private String basedir; // 需要該類加載器直接加載的類檔案的基目錄
	private HashSet<String> dynaclazns; // 需要由該類加載器直接加載的類名

	public CustomClassLoader(String basedir, String[] clazns) {
		super(null); // 指定父類加載器為 null
		this.basedir = basedir;
		dynaclazns = new HashSet<>();
		loadClassByMe(clazns);
	}

	private void loadClassByMe(String[] clazns) {
		for (int i = 0; i < clazns.length; i++) {
			loadDirectly(clazns[i]);
			dynaclazns.add(clazns[i]);
		}
	}

	private Class loadDirectly(String name) {
		Class cls = null;
		StringBuffer sb = new StringBuffer(basedir);
		String classname = name.replace('.', File.separatorChar) + ".class";
		sb.append(File.separator + classname);
		File classF = new File(sb.toString());
		try {
			cls = instantiateClass(name, new FileInputStream(classF),classF.length());
		} catch (FileNotFoundException e) {			
			e.printStackTrace();
		}
		return cls;
	}

	private Class instantiateClass(String name, InputStream fin, long len) {
		byte[] raw = new byte[(int) len];
		try {
			fin.read(raw);
			fin.close();
		} catch (IOException e) {
			e.printStackTrace();
		}

		return defineClass(name, raw, 0, raw.length);
	}

	protected Class loadClass(String name, boolean resolve)
			throws ClassNotFoundException {
		Class cls = null;
		cls = findLoadedClass(name);
		if (!this.dynaclazns.contains(name) && cls == null)
			cls = getSystemClassLoader().loadClass(name);
		if (cls == null)
			throw new ClassNotFoundException(name);
		if (resolve)
			resolveClass(cls);
		return cls;
	}
}

/*
 * 每隔500ms運作一次，不斷加載class
 */
class Multirun implements Runnable {
	public void run() {
		try {
			while (true) {
				// 每次都建立出一個新的類加載器
				// class需要放在自己package名字的檔案夾下
				String url = System.getProperty("user.dir") + "/WebRoot/WEB-INF/classes";  // "D:/projects/testWeb/WebRoot/WEB-INF/classes/zmx/classloader/test/DynamicClass.class";
				CustomClassLoader cl = new CustomClassLoader(url, new String[] { "zmx.classloader.test.DynamicClass" });
				Class cls = cl.loadClass("zmx.classloader.test.DynamicClass");
				Object foo = cls.newInstance();
				// 被調用函數的參數
				Method m = foo.getClass().getMethod("Output", new Class[] {});
				m.invoke(foo, new Object[] {});
				Thread.sleep(500);
			}

		} catch (Exception ex) {
			ex.printStackTrace();
		}
	}
}
           

        使用上述類加載器，加載動态類.

2、DynamicClass.java代碼如下：

package zmx.classloader.test;

public class DynamicClass {

 public static double Darts(int n) 

   { 

     int k = 0; 

     double x = 0.0D;    

     double y = 0.0D;    

     for (int i = 0; i < n; i++) 

     { 

       x = Math.random(); 

       y = Math.random();    

       if (x * x + y * y <= 1.0D) 

         k++; 

     } 

     return 4 * k / n; 

   } 

   // 本熱部署實驗中，上面的Darts函數沒有用到，請忽略 

   public static void Output() {       

     System.out.println("Frist Class Output"); 

   }

}

3、測試類MainTest.java代碼如下：

package zmx.classloader.test;

import java.lang.reflect.Method;

import java.net.URL;

import java.net.URLClassLoader;

public class MainTest {

 public static Method initAddMethod() {

  try {

   Method add = URLClassLoader.class.getDeclaredMethod("addURL",new Class[] { URL.class });

   add.setAccessible(true);

   return add;

  } catch (Exception e) {

   throw new RuntimeException(e);

  }

 }

 public static void main(String[] args) throws Exception{

  // 熱部署測試代碼

  Thread t;

  t = new Thread(new Multirun());

  t.start();

 }

}

    程式運作過程中将DynamicClass中的Output()的輸出修改為 System.out.println("Second Class Output");重新編譯。即 運作時候，程式加載指定檔案夾下的.class，即開始輸出 Frist Class Output ，然後手動将另外一個.class(輸出Second Class Output的.class)覆寫此檔案。程式輸出為 Second Class Output。

輸出結果如下：

Frist Class Output

Frist Class Output

Frist Class Output

Frist Class Output

Frist Class Output

Frist Class Output

Second Class Output

Second Class Output

Second Class Output

Second Class Output