1 import java.io.ByteArrayOutputStream;

 2 import java.io.File;

 3 import java.io.FileInputStream;

 4 import java.io.InputStream;

 5

 6 public class ClassLoaderTest extends ClassLoader {

 7

 8     private String classLoaderName;

 9     private String fileExtension = ".class";

10

11     /**

12      * 默认的情况下，自定义类的加载器会以SystemClassLoader为父类加载器，如果要改变这种机制，调第二种生成方法

13      */

14

15     public ClassLoaderTest(String classLoaderName) {

16         super();

17         this.classLoaderName = classLoaderName;

18     }

19

20     public ClassLoaderTest(ClassLoader classLoader, String classLoaderName) {

21         super(classLoader);

22         this.classLoaderName = classLoaderName;

23     }

24

25     /**

26      * 该方法会在底层调用

27      */

28     @Override

29     protected Class<?> findClass(String className) throws ClassNotFoundException {

30         byte[] data = this.loadClassData(className);

31         return this.defineClass(className, data, 0, data.length);

32     }

33

34     // 在该示例里，不会执行该方法，也就是说，由于双亲委托机制，会由应用类加载器加载

35     // 如果加载的类，不在classpath里，意思就是应用类加载器加载不了，才会由此加载器加载

36     private byte[] loadClassData(String name) {

37

38         byte[] data = null;

39         InputStream is = null;

40         ByteArrayOutputStream baos = null;

41

42         try {

43             this.classLoaderName = this.classLoaderName.replace(".", "/");

44

45             is = new FileInputStream(new File(name + this.fileExtension));

46             baos = new ByteArrayOutputStream();

47

48             int ch = 0;

49

50             while (-1 != (ch = is.read())) {

51                 baos.write(ch);

52             }

53             data = baos.toByteArray();

54

55         } catch (Exception e) {

56             e.printStackTrace();

57         } finally {

58             try {

59                 is.close();

60                 baos.close();

61             } catch (Exception e) {

62                 e.printStackTrace();

63             }

64         }

65

66         return data;

67     }

68

69     public static void test(ClassLoader classLoader) throws Exception {

70         Class<?> clazz = classLoader.loadClass("类名");

71         Object o = clazz.newInstance();

72         System.out.println(o);

73     }

74

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

76         ClassLoaderTest loader1 = new ClassLoaderTest("loader1");

77         test(loader1);

78     }

79

80 }