自定义 Java ClassLoader

首先看一下 java.lang.ClassLoader 方法 loadClass(String name) 方法的源码

public Class loadClass(String name) throws ClassNotFoundException {
        return loadClass(name, false);
}

protected Class loadClass(String name, boolean resolve) throws ClassNotFoundException {
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            Class c = findLoadedClass(name);//检查，class是否已经被加载过了，如果加载过了就不会再加载，防止重复加载。
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                    if (parent != null) {
                        c = parent.loadClass(name, false);//如果没有被加载，则交给父类 ClassLoader 进行加载，java的双亲委派类加载机制
                    } else {
                        c = findBootstrapClassOrNull(name);//如果没有父类加载器，则 直接交给最顶层的 类加载器进行加载。
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                    // from the non-null parent class loader
                }
                if (c == null) {//如果父类加载器，也没有加载到，调用 findClass 方法。
                    // If still not found, then invoke findClass in order
                    // to find the class.
                    long t1 = System.nanoTime();
                    c = findClass(name);//findClass 源码，见下面。
                    // this is the defining class loader; record the stats
                    sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                    sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                    sun.misc.PerfCounter.getFindClasses().increment();
                }
            }
            if (resolve) {
    /**
     * 关于方法 resolveClass 的解释
     * Links the specified class.  This (misleadingly named) method may be
     * used by a class loader to link a class.  If the class c has
     * already been linked, then this method simply returns. Otherwise, the
     * class is linked as described in the "Execution" chapter of
     * The Java™ Language Specification.
     * 

     *
     * @param  c
     *         The class to link
     *
     * @throws  NullPointerException
     *          If c is null.
     *
     * @see  #defineClass(String, byte[], int, int)
     */
                resolveClass(c);
            }
            return c;
        }
}
//什么都没有，就是简单抛出一个 Exception
protected Class findClass(String name) throws ClassNotFoundException {
        throw new ClassNotFoundException(name);
}

通过分析源码，我们知道要自定义 ClassLoader 那么继承 ClassLoader 重写 findClass 方法就行了。

示例代码
import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.nio.ByteBuffer;
import java.nio.channels.Channels;
import java.nio.channels.FileChannel;
import java.nio.channels.WritableByteChannel;
public class MyClassLoader extends ClassLoader {

      @Override
      protected Class findClass(String name) throws ClassNotFoundException {
            Class c = null;
            try {
                  byte[] data = getClassFileBytes(name);
                  c = defineClass(name, data, 0, data.length);//name参数是要加载类的 package
            } catch (Exception e) {
                  e.printStackTrace();
            }
        return c;
      }

      private byte[] getClassFileBytes(String name) throws Exception {
        String path = "D:\\MyEclipse 10\\jd-finance-platform\\AtestJDK1-8\\bin\\";
        String classPath = name.replace(".", "\\") + ".class";
        //class 文件的存放路径
        String classFile = path + classPath;
        FileInputStream fis = new FileInputStream(classFile);
        FileChannel fileC = fis.getChannel();
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        WritableByteChannel outC = Channels.newChannel(baos);
        ByteBuffer buffer = ByteBuffer.allocateDirect(1024);
        while (true) {
            int i = fileC.read(buffer);
            if (i == 0 || i == -1) {
                break;
            }
            buffer.flip();
            outC.write(buffer);
            buffer.clear();
        }
        fis.close();
        return baos.toByteArray();
    }

}

测试代码
public class Test{ 

    public static void main(String[] args) throws Exception{ 
      MyClassLoader myc = new MyClassLoader();
      Class c = myc.loadClass("com.lp.jdk18.JDK18");//字符串是要加载类的 package
      Object obj = c.newInstance();
      System.out.println(obj);
      System.out.println(obj.getClass().getClassLoader());
      System.out.println(obj.getClass().getClassLoader().getParent());
    }

}

输出结果
com.lp.jdk18.JDK18@520b368f
com.lp.classload.MyClassLoader@77f2fbff
sun.misc.Launcher$AppClassLoader@15b94ed3