猫头鹰的深夜翻译：理解java的classloader

Class loadClass(String name, boolean resolve)

% java Foo arg1 arg2

% java CCLRun Foo arg1 arg2

// $Id$
 
import java.io.*;
 
/*
 
A CompilingClassLoader compiles your Java source on-the-fly.  It
checks for nonexistent .class files, or .class files that are older
than their corresponding source code.

*/
 
public class CompilingClassLoader extends ClassLoader
{
  // Given a filename, read the entirety of that file from disk
  // and return it as a byte array.
  private byte[] getBytes( String filename ) throws IOException {
    // Find out the length of the file
    File file = new File( filename );
    long len = file.length();
 
    // Create an array that's just the right size for the file's
    // contents
    byte raw[] = new byte[(int)len];
 
    // Open the file
    FileInputStream fin = new FileInputStream( file );
 
    // Read all of it into the array; if we don't get all,
    // then it's an error.
    int r = fin.read( raw );
    if (r != len)
      throw new IOException( "Can't read all, "+r+" != "+len );
 
    // Don't forget to close the file!
    fin.close();
 
    // And finally return the file contents as an array
    return raw;
  }
 
  // Spawn a process to compile the java source code file
  // specified in the 'javaFile' parameter.  Return a true if
  // the compilation worked, false otherwise.
  private boolean compile( String javaFile ) throws IOException {
    // Let the user know what's going on
    System.out.println( "CCL: Compiling "+javaFile+"..." );
 
    // Start up the compiler
    Process p = Runtime.getRuntime().exec( "javac "+javaFile );
 
    // Wait for it to finish running
    try {
      p.waitFor();
    } catch( InterruptedException ie ) { System.out.println( ie ); }
 
    // Check the return code, in case of a compilation error
    int ret = p.exitValue();
 
    // Tell whether the compilation worked
    return ret==0;
  }
 
  // The heart of the ClassLoader -- automatically compile
  // source as necessary when looking for class files
  public Class loadClass( String name, boolean resolve )
      throws ClassNotFoundException {
 
    // Our goal is to get a Class object
    Class clas = null;
 
    // First, see if we've already dealt with this one
    clas = findLoadedClass( name );
 
    //System.out.println( "findLoadedClass: "+clas );
 
    // Create a pathname from the class name
    // E.g. java.lang.Object => java/lang/Object
    String fileStub = name.replace( '.', '/' );
 
    // Build objects pointing to the source code (.java) and object
    // code (.class)
    String javaFilename = fileStub+".java";
    String classFilename = fileStub+".class";
 
    File javaFile = new File( javaFilename );
    File classFile = new File( classFilename );
 
    //System.out.println( "j "+javaFile.lastModified()+" c "+
    //  classFile.lastModified() );
 
    // First, see if we want to try compiling.  We do if (a) there
    // is source code, and either (b0) there is no object code,
    // or (b1) there is object code, but it's older than the source
    if (javaFile.exists() &&
         (!classFile.exists() ||
          javaFile.lastModified() > classFile.lastModified())) {
 
      try {
        // Try to compile it.  If this doesn't work, then
        // we must declare failure.  (It's not good enough to use
        // and already-existing, but out-of-date, classfile)
        if (!compile( javaFilename ) || !classFile.exists()) {
          throw new ClassNotFoundException( "Compile failed: "+javaFilename );
        }
      } catch( IOException ie ) {
 
        // Another place where we might come to if we fail
        // to compile
        throw new ClassNotFoundException( ie.toString() );
      }
    }
 
    // Let's try to load up the raw bytes, assuming they were
    // properly compiled, or didn't need to be compiled
    try {
 
      // read the bytes
      byte raw[] = getBytes( classFilename );
 
      // try to turn them into a class
      clas = defineClass( name, raw, 0, raw.length );
    } catch( IOException ie ) {
      // This is not a failure!  If we reach here, it might
      // mean that we are dealing with a class in a library,
      // such as java.lang.Object
    }
 
    //System.out.println( "defineClass: "+clas );
 
    // Maybe the class is in a library -- try loading
    // the normal way
    if (clas==null) {
      clas = findSystemClass( name );
    }
 
    //System.out.println( "findSystemClass: "+clas );
 
    // Resolve the class, if any, but only if the "resolve"
    // flag is set to true
    if (resolve && clas != null)
      resolveClass( clas );
 
    // If we still don't have a class, it's an error
    if (clas == null)
      throw new ClassNotFoundException( name );
 
    // Otherwise, return the class
    return clas;
  }
}

import java.lang.reflect.*;
 
/*
 
CCLRun executes a Java program by loading it through a
CompilingClassLoader.
 
*/
 
public class CCLRun
{
  static public void main( String args[] ) throws Exception {
 
    // The first argument is the Java program (class) the user
    // wants to run
    String progClass = args[0];
 
    // And the arguments to that program are just
    // arguments 1..n, so separate those out into
    // their own array
    String progArgs[] = new String[args.length-1];
    System.arraycopy( args, 1, progArgs, 0, progArgs.length );
 
    // Create a CompilingClassLoader
    CompilingClassLoader ccl = new CompilingClassLoader();
 
    // Load the main class through our CCL
    Class clas = ccl.loadClass( progClass );
 
    // Use reflection to call its main() method, and to
    // pass the arguments in.
 
    // Get a class representing the type of the main method's argument
    Class mainArgType[] = { (new String[0]).getClass() };
 
    // Find the standard main method in the class
    Method main = clas.getMethod( "main", mainArgType );
 
    // Create a list containing the arguments -- in this case,
    // an array of strings
    Object argsArray[] = { progArgs };
 
    // Call the method
    main.invoke( null, argsArray );
  }
}

public class Foo
{
  static public void main( String args[] ) throws Exception {
    System.out.println( "foo! "+args[0]+" "+args[1] );
    new Bar( args[0], args[1] );
  }
}

import baz.*;
 
public class Bar
{
  public Bar( String a, String b ) {
    System.out.println( "bar! "+a+" "+b );
    new Baz( a, b );
 
    try {
      Class booClass = Class.forName( "Boo" );
      Object boo = booClass.newInstance();
    } catch( Exception e ) {
      e.printStackTrace();
    }
  }
}

package baz;
 
public class Baz
{
  public Baz( String a, String b ) {
    System.out.println( "baz! "+a+" "+b );
  }
}

public class Boo
{
  public Boo() {
    System.out.println( "Boo!" );
  }
}