【深刻Java虛擬機】二 類加載與雙親委派

https://blog.csdn.net/zhangliangzi/article/details/51338291  -參考

 

雙親委派過程：當一個類加載器收到類加載任務時，當即將任務委派給它的父類加載器去執行，直至委派給最頂層的啓動類加載器爲止。若是父類加載器沒法加載委派給它的類時，將類加載任務退回給它的下一級加載器去執行;除了啓動類加載器之外，每一個類加載器擁有一個父類加載器，用戶的自定義類加載器的父類加載器是AppClassLoader；雙親委派模型能夠保證全限名指定的類，只被加載一次；雙親委派模型不具備強制性約束，是Java設計者推薦的類加載器實現方式；

 

雙親委派模型的源碼實現：

主要體如今ClassLoader的loadClass()方法中，思路很簡單：先檢查是否已經被加載過，若沒有加載則調用父類加載器的loadClass()方法，若父類加載器爲空則默認使用啓動類加載器做爲父類加載器。若是父類加載器加載失敗，拋出ClassNotFoundException異常後，調用本身的findClass()方法進行加載。

 

/**
     * 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:
     *
     * <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
            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
                    sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                    sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                    sun.misc.PerfCounter.getFindClasses().increment();
                }
            }
            if (resolve) {
                resolveClass(c);
            }
            return c;
        }
    }

雙親委派的好處： 

    使用雙親委派模型來組織類加載器之間的關係，有一個很明顯的好處，就是Java類隨着它的類加載器（說白了，就是它所在的目錄）一塊兒具有了一種帶有優先級的層次關係，這對於保證Java程序的穩定運做很重要。例如，類java.lang.Object類存放在JDK\jre\lib下的rt.jar之中，所以不管是哪一個類加載器要加載此類，最終都會委派給啓動類加載器進行加載，這邊保證了Object類在程序中的各類類加載器中都是同一個類。

破壞雙親委派的模型： 

 

 

 

參考：

[1] 《Java虛擬機精講》[2] 《深刻理解Java虛擬機 JVM高級特性與最佳實踐》[3]   http://blog.csdn.net/zhangliangzi/article/details/51338291

[2] tomcat類加載器

[3] 深刻探討 Java 類加載器：https://www.ibm.com/developerworks/cn/java/j-lo-classloader/index.html

[4] 深刻理解 Tomcat（四）Tomcat 類加載器之爲什麼違背雙親委派模型 http://www.javashuo.com/article/p-nqgssvfe-d.html