要點：

1.strut2是依賴於xwork這個包，這一點也struts1與struts2的區別之一。     

2.在strut2中ActionSupport類的Logger物件LOG，它是一個靜態物件，通過日誌工廠LoggerFactory去獲取的。在採用STRUTS2進行開發時，可以讓action類繼承ActionSupport，即可使用Logger進行日誌處理（Logger，包含trace debug info warn error fatal等六個層別的列印）

下面通過Logger物件，瞭解一下Struts2的日誌處理過程

ActionSupport持有LOG物件，原始碼如下：

public class ActionSupport
  implements Action, Validateable, ValidationAware, TextProvider, LocaleProvider, Serializable
{
  protected static Logger LOG = LoggerFactory.getLogger(ActionSupport.class);

......

｝

那麼LoggerFactory方法，getLogger是如何實現的呢？

##########################XWORK包下##########################

首先，LoggerFactory是一個抽象類，它持有一把讀寫鎖lock(ReadWriteLock)，以及一個日誌工廠factory(LoggerFactory).

 private static final ReadWriteLock lock = new ReentrantReadWriteLock();
  private static LoggerFactory factory;

提供兩個靜態方式，獲取Logger物件，一種是通過Class<?> cls，一種是通過類名 字串引數，如Hello.class就用“Hello”，

  public static Logger getLogger(Class<?> cls)
  {
    return getLoggerFactory().getLoggerImpl(cls);
  }

  public static Logger getLogger(String name) {
    return getLoggerFactory().getLoggerImpl(name);
  }
從上面看，實現上是呼叫其自身的實現方法getLoggerImpl，它自身的getLoggerImpl方法如下，都是抽象方法（為什麼？往下看方法getLoggerFactory，就知道

 protected abstract Logger getLoggerImpl(Class<?> paramClass);

  protected abstract Logger getLoggerImpl(String paramString);

#################getLoggerFactory();的實現#####################

在理解getLoggerFactory().getLoggerImpl(cls);這個語句之前，我們先理解一下getLoggerFactory();的實現

protected static LoggerFactory getLoggerFactory() {
    lock.readLock().lock(); //鎖定 讀鎖
    try {
      if (factory != null) {  //工廠是共用的，如果不用空，返回該工廠物件
        LoggerFactory localLoggerFactory = factory;
        return localLoggerFactory;  //如果不用空，返回該工廠物件
      } } finally { lock.readLock().unlock();//解除  讀鎖
    }
    lock.writeLock().lock();  //鎖定  寫鎖;前提，如果當前factory為空，需要建立新的工廠，使用ActionSupport 裡邊的LoggerFactory.getLogger()方法，展開即getLoggerFactory();方法返回值 不為空
    try {
      if (factory == null) {  //important!!!!!!  通過 反射 reflection 獲取commons-logging下的LogFactory物件
        try {
          Class.forName("org.apache.commons.logging.LogFactory");//為什麼呢？
          factory = new CommonsLoggerFactory();//在寫鎖當環境中 建立 CommonsLoggerFactory物件，要用到LogFactory物件 ，詳細程式碼，往下看!
        }
        catch (java.lang.ClassNotFoundException ex) {
          factory = new JdkLoggerFactory();
        }
      }
      ex = factory;
      return ex; } finally { lock.writeLock().unlock(); } throw localObject2;//解除  寫鎖
  }

注：CommonsLoggerFactory 其實是LoggerFactory的子類，只是它是一個非抽象物件，實現了LoggerFactory的下面的兩個實現方法：

 protected abstract Logger getLoggerImpl(Class<?> paramClass);

  protected abstract Logger getLoggerImpl(String paramString);

################建立  CommonsLoggerFactory  物件 的實現################

public class CommonsLoggerFactory extends LoggerFactory
{
  protected Logger getLoggerImpl(Class<?> cls)
  {
    return new CommonsLogger(LogFactory.getLog(cls));
  }

  protected Logger getLoggerImpl(String name)
  {
    return new CommonsLogger(LogFactory.getLog(name));
  }
}

#######COMMONS-LOGGIN包下的###

######################  LogFactory物件getLog方法 ，引數為Class cls 的實現################

  public static Log getLog(Class clazz)
    throws LogConfigurationException
  {
    return getFactory().getInstance(clazz);
  }

在看下commons-logging下的LogFactory類的getFactory()方法的實現之前，

先理解一下LogFactory的靜態載入塊：

  static
  {
    thisClassLoader = getClassLoader(LogFactory.class);  //獲取當前類（LogFactory）的類載入器
    initDiagnostics();
    logClassLoaderEnvironment(LogFactory.class);
    factories = createFactoryStore();
    if (isDiagnosticsEnabled())
      logDiagnostic("BOOTSTRAP COMPLETED");
  }

#################initDiagnostics####################

  private static void initDiagnostics()
  {
    try
    {
      String dest = getSystemProperty("org.apache.commons.logging.diagnostics.dest", null);  //讀取專案根目錄下的commons-loggings.properties檔案裡邊的

//org.apache.commons.logging.diagnostics.dest系統變數

      if (dest == null) {
        return;
      }
    }
    catch (SecurityException ex)
    {
      return;
    }
    String dest;
    if (dest.equals("STDOUT"))
      diagnosticsStream = System.out;
    else if (dest.equals("STDERR"))
      diagnosticsStream = System.err;
    else {//其他情況、根據檔名建立列印流diagnosticsStream（PrintStream）
      try
      {
        FileOutputStream fos = new FileOutputStream(dest, true);
        diagnosticsStream = new PrintStream(fos);
      }
      catch (IOException ex) {
        return;
      }

    }

    String classLoaderName;//初始化classLoaderName類載入器名稱
    try
    {
      ClassLoader classLoader = thisClassLoader;
      String classLoaderName;
      if (thisClassLoader == null)
        classLoaderName = "BOOTLOADER";
      else
        classLoaderName = objectId(classLoader);//objectId返回形式為“該類的完整路徑（如java.lang.String）[email protected]+該類在記憶體中的hashcode編碼”
    }
    catch (SecurityException e)
    {
      String classLoaderName;
      classLoaderName = "UNKNOWN";
    }
    diagnosticPrefix = "[LogFactory from " + classLoaderName + "] ";//定義診斷字首字串
  }
#################logClassLoaderEnvironment#######

###################這個類比較簡單 ，只是載入類載入器，並列印一些資訊####################

##################前提是isDiagnosticsEnabled（）為true,即diagnosticsStream（PrintStream） 物件不為空的情況#############
  private static void logClassLoaderEnvironment(Class clazz)
  {
    if (!isDiagnosticsEnabled()) {
      return;
    }

    try
    {
      logDiagnostic("[ENV] Extension directories (java.ext.dir): " + System.getProperty("java.ext.dir"));
      logDiagnostic("[ENV] Application classpath (java.class.path): " + System.getProperty("java.class.path"));
    } catch (SecurityException ex) {
      logDiagnostic("[ENV] Security setting prevent interrogation of system classpaths.");
    }

    String className = clazz.getName();
    try
    {
      classLoader = getClassLoader(clazz);
    }
    catch (SecurityException ex)
    {
      ClassLoader classLoader;
      logDiagnostic("[ENV] Security forbids determining the classloader for " + className);

      return;
    }
    ClassLoader classLoader;
    logDiagnostic("[ENV] Class " + className + " was loaded via classloader " + objectId(classLoader));

    logHierarchy("[ENV] Ancestry of classloader which loaded " + className + " is ", classLoader);
  }

#################createFactoryStore####################

 private static final Hashtable createFactoryStore() { //這裡返回一個執行緒安全，支援同步的hashtable物件，用於儲存工廠集合

    Hashtable result = null;
    String storeImplementationClass;
    try {

storeImplementationClass = getSystemProperty("org.apache.commons.logging.LogFactory.HashtableImpl", null);//讀取系統配置，在commons-loggin.properties下配置
    }
    catch (SecurityException ex)
    {
      String storeImplementationClass;
      storeImplementationClass = null;
    }

    if (storeImplementationClass == null)//如果沒有配置，讀出預設弱hashtable物件
      storeImplementationClass = "org.apache.commons.logging.impl.WeakHashtable";
    try
    {
      Class implementationClass = Class.forName(storeImplementationClass);//反射獲取該例項返回
      result = (Hashtable)implementationClass.newInstance();
    }
    catch (Throwable t)
    {
      if (!"org.apache.commons.logging.impl.WeakHashtable".equals(storeImplementationClass))
      {
        if (isDiagnosticsEnabled())
        {
          logDiagnostic("[ERROR] LogFactory: Load of custom hashtable failed");
        }
        else
        {
          System.err.println("[ERROR] LogFactory: Load of custom hashtable failed");
        }
      }
    }
    if (result == null) {
      result = new Hashtable();
    }
    return result;
  }

#####################現在靜態塊程式碼已經分析完了，只是一個靜態載入，比較簡單###接下繼續前面的Log#################

###############LogFactory類的getFactory()方法的實現##################################

  首先呼叫getContextClassLoaderInternal()方法，通過AccessController物件呼叫directGetContextClassLoader

#################directGetContextClassLoader 的程式碼如下##########反射####################################

Method method = Thread.class.getMethod("getContextClassLoader", (Class[])null);
classLoader = (ClassLoader)method.invoke(Thread.currentThread(), (Object[])null);

#########################如果classLoader為空，則返回當前類的載入器########

classLoader = getClassLoader(LogFactory.class);

  接下來，通過classLoader類載入器作為引數，獲取快取工廠

factory  = getCachedFactory（classLoader）