自定义一个类加载器

 1 protected synchronized Class<?> loadClass(String name, boolean resolve)
 2     throws ClassNotFoundException
 3     {
 4     // First, check if the class has already been loaded
 5     Class c = findLoadedClass(name);
 6     if (c == null) {
 7         try {
 8         if (parent != null) {
 9             c = parent.loadClass(name, false);
10         } else {
11             c = findBootstrapClass0(name);
12         }
13         } catch (ClassNotFoundException e) {
14             // If still not found, then invoke findClass in order
15             // to find the class.
16             c = findClass(name);
17         }
18     }
19     if (resolve) {
20         resolveClass(c);
21     }
22     return c;
23     }

protected Class<?> findClass(String name) throws ClassNotFoundException {
    throw new ClassNotFoundException(name);
    }

 1 package com.xrq.classloader;
 2 
 3 public class Person
 4 {
 5     private String name;
 6     
 7     public Person()
 8     {
 9         
10     }
11     
12     public Person(String name)
13     {
14         this.name = name;
15     }
16     
17     public String getName()
18     {
19         return name;
20     }
21     
22     public void setName(String name)
23     {
24         this.name = name;
25     }
26     
27     public String toString()
28     {
29         return "I am a person, my name is " + name;
30     }
31 }

public class MyClassLoader extends ClassLoader
{
    public MyClassLoader()
    {
        
    }
    
    public MyClassLoader(ClassLoader parent)
    {
        super(parent);
    }
    
    protected Class<?> findClass(String name) throws ClassNotFoundException
    {
        File file = getClassFile(name);
        try
        {
            byte[] bytes = getClassBytes(file);
            Class<?> c = this.defineClass(name, bytes, 0, bytes.length);
            return c;
        } 
        catch (Exception e)
        {
            e.printStackTrace();
        }
        
        return super.findClass(name);
    }
    
    private File getClassFile(String name)
    {
        File file = new File("D:/Person.class");
        return file;
    }
    
    private byte[] getClassBytes(File file) throws Exception
    {
        // 这里要读入.class的字节，因此要使用字节流
        FileInputStream fis = new FileInputStream(file);
        FileChannel fc = fis.getChannel();
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        WritableByteChannel wbc = Channels.newChannel(baos);
        ByteBuffer by = ByteBuffer.allocate(1024);
        
        while (true)
        {
            int i = fc.read(by);
            if (i == 0 || i == -1)
                break;
            by.flip();
            wbc.write(by);
            by.clear();
        }
        
        fis.close();
        
        return baos.toByteArray();
    }
}

 1 public class TestMyClassLoader
 2 {
 3     public static void main(String[] args) throws Exception
 4     {
 5         MyClassLoader mcl = new MyClassLoader();        
 6         Class<?> c1 = Class.forName("com.xrq.classloader.Person", true, mcl); 
 7         Object obj = c1.newInstance();
 8         System.out.println(obj);
 9         System.out.println(obj.getClass().getClassLoader());
10     }
11 }

I am a person, my name is null
com.xrq.classloader.MyClassLoader@5d888759

 1 public InputStream getResourceAsStream(String name) {
 2     name = resolveName(name);
 3     ClassLoader cl = getClassLoader0();
 4     if (cl==null) {
 5         // A system class.
 6         return ClassLoader.getSystemResourceAsStream(name);
 7     }
 8     return cl.getResourceAsStream(name);
 9 }
10 
11 private String resolveName(String name) {
12     if (name == null) {
13         return name;
14     }
15     if (!name.startsWith("/")) {
16         Class c = this;
17         while (c.isArray()) {
18             c = c.getComponentType();
19         }
20         String baseName = c.getName();
21         int index = baseName.lastIndexOf('.');
22         if (index != -1) {
23             name = baseName.substring(0, index).replace('.', '/')
24                 +"/"+name;
25         }
26     } else {
27         name = name.substring(1);
28     }
29     return name;
30 }