Custom JVM class loader running case in complex class analysis

A custom class loader running case in complex class analysis

1, previously created using a class loader

class MyTest16 the extends ClassLoader {public 

    Private String className; 

    // directory 
     Private String path; 

    Private Final fileExtension String = ".class"; 

    public MyTest16 (String classLoadName) { 
        Super (); // the system class loader class loader as parent loader 
        this.className = classLoadName; 
    } 

    public MyTest16 (ClassLoader parent, String classLoadName) { 
        Super (parent); // displays the parent class loader loader is 
        this.className = classLoadName; 
    } 

    public void setPath ( path String) { 
        this.path = path; 
    } 

    @Override 
    public String toString () {  
        return "[" + this.className + "]";
    } 

    @Override
    protected Class<?> findClass(String clasName) throws ClassNotFoundException {
        System.out.println("findClass invoked:" + clasName);
        System.out.println("class loader name: " + this.className);
        byte[] data = this.loadClassData(clasName);
        return  this.defineClass(clasName,data, 0, data.length);
    }

    private byte[] loadClassData(String className){
        InputStream is = null;
        byte[] data = null;
        ByteArrayOutputStream baos = null;

        try{
            className = className.replace(".","//");
            //System.out.println("className:" +this.className);
            is = new FileInputStream(new File(this.path + className + this.fileExtension));
            baos = new ByteArrayOutputStream();
            int ch = 0;
            while ( -1 != (ch = is.read())){
                baos.write(ch);
            }
            data = baos.toByteArray();

        }catch (Exception ex){
            ex.printStackTrace();
        }finally {
            try {
                is.close();
                baos.close();
            }catch (Exception ex){
                ex.printStackTrace();
            }
        }

        return  data;

    }

   


}

　　

2. Create a class MyCat

public class MyCat {
    public MyCat(){
        System.out.println("MyCat is loaded by:" + this.getClass().getClassLoader());
    }
}

　　

3. Create MySample class

public class MySample {

    public MySample(){
        System.out.println("MySample is loaded by:" + this.getClass().getClassLoader());
        new MyCat();

    }
}

　　

4, create a test class

{class MyTest17 public 
    public static void main (String [] args) throws Exception { 
        MyTest16 loader1 new new MyTest16 = ( "loader1"); 

        <?> = loader1.loadClass clazz Class ( "com.example.jvm.classloader.MySample") ; 

        System.out.println ( "class:" + clazz.hashCode ()); 

        // if commented diverted, then MySample not instantiate objects, i.e. MySample constructor will not be called 
        // therefore not instantiated myCat objects, i.e. no myCat actively used here does not aggravate Class myCat 
        Object = clazz.newInstance Object (); 



    } 
}

　　

Print results

class:1735600054
MySample is loaded by:sun.misc.Launcher$AppClassLoader@18b4aac2
MyCat is loaded by:sun.misc.Launcher$AppClassLoader@18b4aac2

　　

Increase -XX: + printing results after TraceClassLoading

 

 

If you remove the Object object = clazz.newInstance ();

Print results

Note: If commented Object object = clazz.newInstance (); the row, MySample not instantiate objects, i.e. MySample constructor will not be called

         MyCat therefore does not instantiate the object, i.e. no myCat actively used here does not load MyCat Class.

         This example illustrates but is not preloaded MyCat

 

 

Second, a transformation of the Code

In one basis, to test a new class

{class MyTest17_1 public 
    public static void main (String [] args) throws Exception { 
        MyTest16 loader1 new new MyTest16 = ( "loader1"); 
        loader1.setPath:; ( "D / TEMP /") 
        <?> = loader1.loadClass Class clazz ( "com.example.jvm.classloader.MySample"); 

        System.out.println ( "class:" + clazz.hashCode ()); 

        // comment out the line, and it does not instantiate objects MySample, i.e. MySample constructor will not be called 
        // myCat therefore does not instantiate the object, i.e. no myCat actively used here does not load Class myCat 
        Object = clazz.newInstance Object (); 



    } 
}

　　A method which adds loader1.setPath ( "D: / temp / ");

 

MyCat.class and then cut to MySample.class D: / lower temp / directory, following two figures

 

 

Print Results:

findClass invoked:com.example.jvm.classloader.MySample
class loader name: loader1
class:2133927002
MySample is loaded by:[loader1]
findClass invoked:com.example.jvm.classloader.MyCat
class loader name: loader1
MyCat is loaded by:[loader1]