虚拟机类加载机制——类加载器

 1 package day_14_classloader;
 2 
 3 import java.io.IOException;
 4 import java.io.InputStream;
 5 
 6 /**
 7  * @author turbo
 8  *
 9  * 2016年9月19日
10  */
11 public class ClassLoaderTest {
12 
13     /**
14      * @param args
15      */
16     public static void main(String[] args) throws Exception{
17         ClassLoader myLoader = new ClassLoader() {
18 
19             @Override
20             public Class<?> loadClass(String name)
21                     throws ClassNotFoundException {
22                 try {
23                     String fileName = name.substring(name.lastIndexOf(".") + 1) + ".class";
24                     InputStream is = getClass().getResourceAsStream(fileName);
25                     if (is == null){
26                         return super.loadClass(name);
27                     }
28                     byte[] b = new byte[is.available()];
29                     is.read(b);
30                     return defineClass(name, b, 0, b.length);
31                 } catch (IOException e) {
32                     throw new ClassNotFoundException();
33                 }
34             }
35         };
36         
37         Object obj = myLoader.loadClass("day_14_classloader.ClassLoaderTest").newInstance();
38         System.out.println(obj.getClass());
39         System.out.println(obj instanceof day_14_classloader.ClassLoaderTest);
40     }
41 
42 }

 1 /**
 2 *
 3 * 此方法和书中的方法略有不同，书中方法为protected synchronized Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException
 4 * 估计是版本的原因，未考证。
 5 */
 6 protected Class<?> loadClass(String name, boolean resolve)
 7         throws ClassNotFoundException
 8     {
 9         synchronized (getClassLoadingLock(name)) {
10             // 首先, 调用findLoadedClass方法检查请求的类是否已经被加载过了
11             Class c = findLoadedClass(name);
12             if (c == null) {    
13                 long t0 = System.nanoTime();
14                 try {
15                     if (parent != null) {
16                         c = parent.loadClass(name, false);  //有父类加载器，则调用父类加载器
17                     } else {
18                         c = findBootstrapClassOrNull(name);     //没有父类加载器，即调用启动类加载器
19                     }
20                 } catch (ClassNotFoundException e) {
21                     //如果父类抛出ClassNotFoundException异常，说明父类加载器无法完成加载请求
22                 }
23 
24                 if (c == null) {
25                     //如果父类加载器无法加载器的时候调用本身的findClass方法来进行加载
26                     long t1 = System.nanoTime();
27                     c = findClass(name);
28 
29                     // this is the defining class loader; record the stats
30                     sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
31                     sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
32                     sun.misc.PerfCounter.getFindClasses().increment();
33                 }
34             }
35             if (resolve) {
36                 resolveClass(c);
37             }
38             return c;
39         }
40     }