自定义一个类加载器
1 import java.io.ByteArrayOutputStream; 2 import java.io.File; 3 import java.io.FileInputStream; 4 import java.nio.ByteBuffer; 5 import java.nio.channels.Channels; 6 import java.nio.channels.FileChannel; 7 import java.nio.channels.WritableByteChannel; 8 9 /** 10 * 自定义类加载器 11 * @ClassName MyClassLoader 12 * @Author xxx 13 * @Version V1.0 14 **/ 15 public class MyClassLoader extends ClassLoader { 16 17 private File fileObject; 18 19 public void setFileObject(File fileObject) { 20 this.fileObject = fileObject; 21 } 22 23 public File getFileObject() { 24 return fileObject; 25 } 26 27 @Override 28 protected Class<?> findClass(String name) throws ClassNotFoundException { 29 try { 30 byte[] data = getClassFileBytes(this.fileObject); 31 return defineClass(name, data, 0, data.length); 32 } catch (Exception e) { 33 e.printStackTrace(); 34 return null; 35 } 36 37 } 38 39 /** 40 * 从文件中读取去class文件 41 * 42 * @throws Exception 43 */ 44 private byte[] getClassFileBytes(File file) throws Exception { 45 //采用NIO读取 46 FileInputStream fis = new FileInputStream(file); 47 FileChannel fileC = fis.getChannel(); 48 ByteArrayOutputStream baos = new ByteArrayOutputStream(); 49 WritableByteChannel outC = Channels.newChannel(baos); 50 ByteBuffer buffer = ByteBuffer.allocateDirect(1024); 51 while (true) { 52 int i = fileC.read(buffer); 53 if (i == 0 || i == -1) { 54 break; 55 } 56 buffer.flip(); 57 outC.write(buffer); 58 buffer.clear(); 59 } 60 fis.close(); 61 return baos.toByteArray(); 62 } 63 64 }
重点：需要继承
ClassLoader 类，并重写
findClass()方法。注意：官方不建议重写loadClass()方法，可能会破坏双亲委托机制。
使用这个自定义类加载器时，先
1 MyClassLoader myClassLoader = new MyClassLoader(); 2 myClassLoader.setFileObject(new File("此处填需要加载的class文件全路径")); 3 Class<?> test = myClassLoader.findClass("Test");
4 System.out.println(test.getClassLoader());
这个test的类加载器就是自定义的
MyClassLoader ！
