类加载机制及类加载器加载Class流程

(3)、若是父类加载器存在，则请求使用父类加载器去载入目标类，若是成功则返回对应的java.lang.Class对象，失败则使用当前类加载器尝试寻找Class文件(从与此ClassLoader相关的类路径中寻找），若是找到则从文件中载入class，成功则返回对应的java.lang.Class对象，若是找不到则抛出ClassNotFoundException.

流程图以下


ClassLoader类的loadClass方法源码以下。

/**
     * Loads the class with the specified <a href="#name">binary name</a>.  The
     * default implementation of this method searches for classes in the
     * following order:
     *
     * <p><ol>
     *
     *   <li><p> Invoke {@link #findLoadedClass(String)} to check if the class
     *   has already been loaded.  </p></li>
     *
     *   <li><p> Invoke the {@link #loadClass(String) <tt>loadClass</tt>} method
     *   on the parent class loader.  If the parent is <tt>null</tt> the class
     *   loader built-in to the virtual machine is used, instead.  </p></li>
     *
     *   <li><p> Invoke the {@link #findClass(String)} method to find the
     *   class.  </p></li>
     *
     * </ol>
     *
     * <p> If the class was found using the above steps, and the
     * <tt>resolve</tt> flag is true, this method will then invoke the {@link
     * #resolveClass(Class)} method on the resulting <tt>Class</tt> object.
     *
     * <p> Subclasses of <tt>ClassLoader</tt> are encouraged to override {@link
     * #findClass(String)}, rather than this method.  </p>
     *
     * <p> Unless overridden, this method synchronizes on the result of
     * {@link #getClassLoadingLock <tt>getClassLoadingLock</tt>} method
     * during the entire class loading process.
     *
     * @param  name
     *         The <a href="#name">binary name</a> of the class
     *
     * @param  resolve
     *         If <tt>true</tt> then resolve the class
     *
     * @return  The resulting <tt>Class</tt> object
     *
     * @throws  ClassNotFoundException
     *          If the class could not be found
     */
    protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded缓存中是否存在此类
            Class c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                    if (parent != null) {//父类加载器存在，使用父类加载器载入目标类
                        c = parent.loadClass(name, false);
                    } else {//父类加载器不存在，则使用根类加载器载入目标类
                        c = findBootstrapClassOrNull(name);//此方法为native方法。
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                    // from the non-null parent class loader
                    // 加载失败则抛出ClassNotFoundException
                }

                if (c == null) {//父类加载器或根类加载器均未加载成功，则使用当前类加载器寻找Class文件，ClassLoader容许子类重写findClass方法
                    // 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
                    sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                    sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                    sun.misc.PerfCounter.getFindClasses().increment();
                }
            }
            if (resolve) {
                resolveClass(c);
            }
            return c;
        }
    }