详解Android类加载ClassLoader

//ClassLoader.java
protected Class<?> loadClass(String name, boolean resolve)
  throws ClassNotFoundException
{
    // First, check if the class has already been loaded
    Class c = findLoadedClass(name);
    if (c == null) {
      long t0 = System.nanoTime();
      try {
        if (parent != null) {
          c = parent.loadClass(name, false);
        } else {
          c = findBootstrapClassOrNull(name);
        }
      } catch (ClassNotFoundException e) {
        // ClassNotFoundException thrown if class not found
        // from the non-null parent class loader
      }

      if (c == null) {
        // If still not found, then invoke findClass in order
        // to find the class.
        long t1 = System.nanoTime();
        c = findClass(name);

        // this is the defining class loader; record the stats
      }
    }
    return c;
}

public class PathClassLoader extends BaseDexClassLoader {
  /**
   * Creates a {@code PathClassLoader} that operates on a given list of files
   * and directories. This method is equivalent to calling
   * {@link #PathClassLoader(String, String, ClassLoader)} with a
   * {@code null} value for the second argument (see description there).
   *
   * @param dexPath the list of jar/apk files containing classes and
   * resources, delimited by {@code File.pathSeparator}, which
   * defaults to {@code ":"} on Android
   * @param parent the parent class loader
   */
  public PathClassLoader(String dexPath, ClassLoader parent) {
    super(dexPath, null, null, parent);
  }

  /**
   * Creates a {@code PathClassLoader} that operates on two given
   * lists of files and directories. The entries of the first list
   * should be one of the following:
   *
   * <ul>
   * <li>JAR/ZIP/APK files, possibly containing a "classes.dex" file as
   * well as arbitrary resources.
   * <li>Raw ".dex" files (not inside a zip file).
   * </ul>
   *
   * The entries of the second list should be directories containing
   * native library files.
   *
   * @param dexPath the list of jar/apk files containing classes and
   * resources, delimited by {@code File.pathSeparator}, which
   * defaults to {@code ":"} on Android
   * @param librarySearchPath the list of directories containing native
   * libraries, delimited by {@code File.pathSeparator}; may be
   * {@code null}
   * @param parent the parent class loader
   */
  public PathClassLoader(String dexPath, String librarySearchPath, ClassLoader parent) {
    super(dexPath, null, librarySearchPath, parent);
  }
}

//Class.java
public static Class<?> forName(String className)
      throws ClassNotFoundException {
  return forName(className, true, VMStack.getCallingClassLoader());
}

//dalvik.system.VMStack
/**
 * Returns the defining class loader of the caller's caller.
 *
 * @return the requested class loader, or {@code null} if this is the
 *     bootstrap class loader.
 */
@FastNative
native public static ClassLoader getCallingClassLoader();

public class DexClassLoader extends BaseDexClassLoader {
  /**
   * Creates a {@code DexClassLoader} that finds interpreted and native
   * code. Interpreted classes are found in a set of DEX files contained
   * in Jar or APK files.
   *
   * <p>The path lists are separated using the character specified by the
   * {@code path.separator} system property, which defaults to {@code :}.
   *
   * @param dexPath the list of jar/apk files containing classes and
   *   resources, delimited by {@code File.pathSeparator}, which
   *   defaults to {@code ":"} on Android
   * @param optimizedDirectory directory where optimized dex files
   *   should be written; must not be {@code null}
   * @param librarySearchPath the list of directories containing native
   *   libraries, delimited by {@code File.pathSeparator}; may be
   *   {@code null}
   * @param parent the parent class loader
   */
  public DexClassLoader(String dexPath, String optimizedDirectory,
      String librarySearchPath, ClassLoader parent) {
    super(dexPath, new File(optimizedDirectory), librarySearchPath, parent);
  }
}