最近学习JVM的时候,对类的加载原理有了一定的认识,但是涉及到具体例子时还是没有分析清楚,重写loadClass()方法和重写findClass()方法。找到一篇不错的博客,该博主从对问题的思考到源码跟踪,以及测试都是非常不错的。
转载自:https://www.cnblogs.com/pfxiong/p/4118445.html
//示例:
2 package com.csair.soc;
3
4 import java.io.IOException;
5 import java.io.InputStream;
6
7 public class MyClassLoader1 extends ClassLoader{
8 @Override
9 public Class> loadClass(String name) throws ClassNotFoundException{
10 try{
11 String fileName = name.substring(name.lastIndexOf( ".")+1) + ".class";
12 InputStream is = this.getClass().getResourceAsStream(fileName);
13 byte[] b = new byte[is.available()];
14 is.read(b);
15 return defineClass(name, b, 0, b.length );
16 } catch(IOException e){
17 throw new ClassNotFoundException(name);
18 }
19 }
20 }
21
22
23 package com.csair.soc;
24 public class ClassLoaderTest {
25 public static void main(String[] args) throws InstantiationException, IllegalAccessException, ClassNotFoundException {
26 MyClassLoader1 myLoader = new MyClassLoader1();
27 Object obj = myLoader.loadClass("com.csair.soc.ClassLoaderTest" ).newInstance();
28 System. out.println(obj.getClass());
29 System. out.println(obj.getClass().getClassLoader());
30 System. out.println(obj instanceof com.csair.soc.ClassLoaderTest);
31 }
32 }
输出结果?
Exception in thread "main" java.lang.NullPointerException
at com.csair.soc.MyClassLoader1.loadClass( MyClassLoader1.java:13)
at java.lang.ClassLoader.defineClass1( Native Method)
at java.lang.ClassLoader.defineClassCond( ClassLoader.java:631)
at java.lang.ClassLoader.defineClass( ClassLoader.java:615)
at java.lang.ClassLoader.defineClass( ClassLoader.java:465)
at com.csair.soc.MyClassLoader1.loadClass( MyClassLoader1.java:15)
at com.csair.soc.ClassLoaderTest.main( ClassLoaderTest.java:7)
为什么在自定义的MyClassLoader1中Override loadClass会失败?ClassLoaderTest文件在当前目录下,为什么还会报空指针异常?
在loadClass下,添加以下代码做测试。
System.out.println(name);
结果:
com.csair.soc.ClassLoaderTest
java.lang.Object
Exception in thread "main" java.lang.NullPointerException
at com.csair.soc.MyClassLoader1.loadClass( MyClassLoader1.java:13)
at java.lang.ClassLoader.defineClass1( Native Method)
at java.lang.ClassLoader.defineClassCond( ClassLoader.java:631)
at java.lang.ClassLoader.defineClass( ClassLoader.java:615)
at java.lang.ClassLoader.defineClass( ClassLoader.java:465)
at com.csair.soc.MyClassLoader1.loadClass( MyClassLoader1.java:15)
at com.csair.soc.ClassLoaderTest.main( ClassLoaderTest.java:7)
为什么要加载两次?要加载的类是ClassLoaderTest,为什么还要加载java.lang.Object?
带着问题,断点跟踪一下:loadClass
执行MyClassLoader1 的defineClass(name, b, 0, b.length );方法时,其调用顺序如下:
defineClass(name, b, 0, b.length );
2
3 ---> ClassLoader.class
4 protected final Class> defineClass(String name, byte[] b , int off, int len)
5 throws ClassFormatError
6 {
7 return defineClass(name, b, off, len, null);
8 }
9
10 -->
11
12 protected final Class> defineClass(String name, byte[] b, int off, int len,
13 ProtectionDomain protectionDomain)
14 throws ClassFormatError
15 {
16 return defineClassCond(name, b, off, len, protectionDomain, true);
17 }
18
19 --->
20
21 private final Class> defineClassCond(String name,
22 byte[] b, int off, int len,
23 ProtectionDomain protectionDomain,
24 boolean verify)
25 throws ClassFormatError
26 {
27 protectionDomain = preDefineClass(name, protectionDomain);
28
29 Class c = null;
30 String source = defineClassSourceLocation(protectionDomain);
31
32 try {
33 c = defineClass1(name, b, off, len, protectionDomain, source,
34 verify);
35 } catch (ClassFormatError cfe) {
36 c = defineTransformedClass(name, b, off, len, protectionDomain, cfe,
37 source, verify);
38 }
defineClass1也就是提示程序出错的位置
看看defineClass1方法是什么?在ClassLoader中,定义如下:
private native Class defineClass1(String name, byte[] b, int off, int len,
ProtectionDomain pd, String source,
boolean verify);
也就是说defineClass1是native方法,继续跟踪代码,就发现又调用到自定义的loadClass方法中,此时传入的参数则变成了java.lang.Object。
根据这些可以猜测,类的加载,会将其所有的父类都加载一遍,直到java.lang.Object。
为了验证,这个猜想,写出以下示例。
public class MyClassLoader1 extends ClassLoader{
@Override
public Class> loadClass(String name) throws ClassNotFoundException{
try{
System. out.println(name);
String fileName = name.substring(name.lastIndexOf( ".")+1) + ".class";
InputStream is = this.getClass().getResourceAsStream(fileName);
byte[] b = new byte[is.available()];
is.read(b);
return defineClass(name, b, 0, b.length );
} catch(IOException e){
throw new ClassNotFoundException(name);
}
}
}
public static void main(String[] args) throws InstantiationException, IllegalAccessException, ClassNotFoundException {
MyClassLoader1 myLoader = new MyClassLoader1();
Object obj = myLoader.loadClass("com.csair.soc.SubSample" ).newInstance();
}
SubSample有父类Sample。
输出结果:
com.csair.soc.SubSample
com.csair.soc.Sample
java.lang.Object
Exception in thread "main" java.lang.NullPointerException
at com.csair.soc.MyClassLoader1.loadClass( MyClassLoader1.java:13 )
at java.lang.ClassLoader.defineClass1( Native Method )
at java.lang.ClassLoader.defineClassCond( ClassLoader.java:631 )
at java.lang.ClassLoader.defineClass( ClassLoader.java:615 )
at java.lang.ClassLoader.defineClass( ClassLoader.java:465 )
at com.csair.soc.MyClassLoader1.loadClass( MyClassLoader1.java:15 )
at com.csair.soc.ClassLoaderTest.main( ClassLoaderTest.java:7 )
测试结果和猜测的一样,ClassLoader在加载类的同时,会通过native方法defineClass1,将其所有的父类都加载。当ClassLoader加载父类时,由于loadClass方法被重写,defineClass1会调用自定义的classLoader方法加载父类,因此出现以上错误。过程如下:
为了解决这个问题,可以使用以下方式,修改loadClass,当找不到类文件时,使用父类的ClassLoader试试。
@Override
public Class> loadClass(String name) throws ClassNotFoundException{
try{
String fileName = name.substring(name.lastIndexOf( ".")+1) + ".class";
InputStream is = this.getClass().getResourceAsStream(fileName);
if(is == null ){
return super .loadClass(name);
}
byte[] b = new byte[is.available()];
is.read(b);
return defineClass(name, b, 0, b.length );
} catch(IOException e){
throw new ClassNotFoundException(name);
}
}
但最好的办法是不重写loadClass方法,而是重写findClass方法,同样可以达到目的。
这点在ClassLoader的loadClass方法的注释中有提及
*
Subclasses of ClassLoader are encouraged to override {@link
* #findClass(String)}, rather than this method.
重写findClass后,代码如下:
@Override
public Class> findClass(String name) throws ClassNotFoundException{
try{
String fileName = name.substring(name.lastIndexOf( ".")+1) + ".class";
InputStream is = this.getClass().getResourceAsStream(fileName);
byte[] b = new byte[is.available()];
is.read(b);
return defineClass(name, b, 0, b.length );
} catch(IOException e){
throw new ClassNotFoundException(name);
}
}
输出结果:
class com.csair.soc.ClassLoaderTest
com.csair.soc.MyClassLoader@bfc8e0
false
可以看到类的加载是使用自定义的类加载器,在判断obj instanceof com.csair.soc.ClassLoaderTest时,由于默认的com.csair.soc.ClassLoaderTest使用的是系统类加载器,因此输出为false。
当然,也只有在父类加载器找不到类文件的时候,才会调用子类的findClass方法去寻找类文件。