自定义ClassLoader

冷不防 2024-02-17 16:41 39阅读 0赞

JVM在加载类的时候，都是通过ClassLoader的loadClass()方法来加载class的，loadClass(String name)方法：

Java代码

1.  public Class<?> loadClass(String name) throws ClassNotFoundException \{
2.  return loadClass(name, false);
3.  \}

loadClass(String name)方法再调用loadClass(String name, boolean resolve)方法:  
\- name - 类的二进制名称  
\- resolve - 如果该参数为 true，则分析这个类

Java代码

1.  protected synchronized Class<?> loadClass(String name, boolean resolve)
2.  throws ClassNotFoundException
3.  \{
4.  // First, check if the class has already been loaded 
5.  //JVM 规范规定ClassLoader可以在缓存保留它所加载的Class，如果一个Class已经被加载过，则直接从缓存中获取 
6.  Class c = findLoadedClass(name);
7.  if (c == null) \{
8.  try \{
9.  if (parent != null) \{
10. c = parent.loadClass(name, false);
11. \} else \{
12. c = findBootstrapClass0(name);
13. \}
14. \} catch (ClassNotFoundException e) \{
15. // If still not found, then invoke findClass in order 
16. // to find the class. 
17. c = findClass(name);
18. \}
19. \}
20. if (resolve) \{
21. resolveClass(c);
22. \}
23. return c;
24. \}

如果ClassLoader并没有加载这个class，则调用findBootstrapClass0:

Java代码

1.  private Class findBootstrapClass0(String name)
2.  throws ClassNotFoundException
3.  \{
4.  check();
5.  if (!checkName(name))
6.  throw new ClassNotFoundException(name);
7.  return findBootstrapClass(name);
8.  \}

该方法会调用check()方法来判断这个类是否已经初始化，并且通过checkName(name)来判断由name指定的这个类是否存在  
最后调用findBootstrapClass(name):

Java代码

1.  private native Class findBootstrapClass(String name)
2.  throws ClassNotFoundException;

而这个findBootstrapClass方法是一个native方法，这是我们的root loader，这个载入方法并非是由JAVA所写，而是C++写的，它会最终调用JVM中的原生findBootstrapClass方法来完成类的加载。  
  
如果上面两个都找不到，则使用findClass(name)来查找指定类名的Class:

Java代码

1.  protected Class<?> findClass(String name) throws ClassNotFoundException \{
2.  throw new ClassNotFoundException(name);
3.  \}

**JDK5.0中的说明：**  
使用指定的二进制名称查找类。此方法应该被类加载器的实现重写，该实现按照委托模型来加载类。在通过父类加载器检查所请求的类后，此方法将被 loadClass 方法调用。默认实现抛出一个 ClassNotFoundException。  
所以，我们在自定义类中，只需要重写findClass()即可。  
MyClassLoader类：

Java代码

1.  public class MyClassLoader extends ClassLoader \{
2.  private String fileName;
3.  
4.  public MyClassLoader(String fileName) \{
5.  this.fileName = fileName;
6.  \}
7.  
8.  protected Class<?> findClass(String className) throws ClassNotFoundException \{
9.  Class clazz = this.findLoadedClass(className);
10. if (null == clazz) \{
11. try \{
12. String classFile = getClassFile(className);
13. FileInputStream fis = new FileInputStream(classFile);
14. FileChannel fileC = fis.getChannel();
15. ByteArrayOutputStream baos = new
16. ByteArrayOutputStream();
17. WritableByteChannel outC = Channels.newChannel(baos);
18. ByteBuffer buffer = ByteBuffer.allocateDirect(1024);
19. while (true) \{
20. int i = fileC.read(buffer);
21. if (i == 0 || i == -1) \{
22. break;
23. \}
24. buffer.flip();
25. outC.write(buffer);
26. buffer.clear();
27. \}
28. fis.close();
29. byte\[\] bytes = baos.toByteArray();
30. 
31. clazz = defineClass(className, bytes, 0, bytes.length);
32. \} catch (FileNotFoundException e) \{
33. e.printStackTrace();
34. \} catch (IOException e) \{
35. e.printStackTrace();
36. \}
37. \}
38. return clazz;
39. \}
40. private byte\[\] loadClassBytes(String className) throws
41. ClassNotFoundException \{
42. try \{
43. String classFile = getClassFile(className);
44. FileInputStream fis = new FileInputStream(classFile);
45. FileChannel fileC = fis.getChannel();
46. ByteArrayOutputStream baos = new
47. ByteArrayOutputStream();
48. WritableByteChannel outC = Channels.newChannel(baos);
49. ByteBuffer buffer = ByteBuffer.allocateDirect(1024);
50. while (true) \{
51. int i = fileC.read(buffer);
52. if (i == 0 || i == -1) \{
53. break;
54. \}
55. buffer.flip();
56. outC.write(buffer);
57. buffer.clear();
58. \}
59. fis.close();
60. return baos.toByteArray();
61. \} catch (IOException fnfe) \{
62. throw new ClassNotFoundException(className);
63. \}
64. \}
65. private String getClassFile(String name) \{
66. StringBuffer sb = new StringBuffer(fileName);
67. name = name.replace('.', File.separatorChar) + ".class";
68. sb.append(File.separator + name);
69. return sb.toString();
70. \}
71. \}

该类中通过调用defineClass(String name, byte\[\] b, int off, int len)方法来定义一个类：

Java代码

1.  protected final Class<?> defineClass(String name, byte\[\] b, int off, int len)
2.  throws ClassFormatError
3.  \{
4.  return defineClass(name, b, off, len, null);
5.  \}

**注：**MyClassLoader加载类时有一个局限，必需指定 **.class**文件，而不能指定 **.jar**文件。MainClassLoader类：

Java代码

1.  public class MainClassLoader \{
2.  public static void main(String\[\] args) \{
3.  try \{
4.  MyClassLoader tc = new MyClassLoader("F:\\\\OpenLib\\\\");
5.  Class c = tc.findClass("Test");
6.  c.newInstance();
7.  \} catch (ClassNotFoundException e) \{
8.  e.printStackTrace();
9.  \} catch (IllegalAccessException e) \{
10. e.printStackTrace();
11. \} catch (InstantiationException e) \{
12. e.printStackTrace();
13. \}
14. \}
15. \}

最后是一个简单的Test测试类：

Java代码

1.  public class Test
2.  \{
3.  public Test() \{
4.  System.out.println("Test");
5.  \}
6.  public static void main(String\[\] args) \{
7.  System.out.println("Hello World");
8.  \}
9.  \}