前言：
本文是基于 ClassLoader雙親委派機(jī)制源碼分析 了解過(guò)正統(tǒng)JDK類加載機(jī)制及其實(shí)現(xiàn)原理的基礎(chǔ)上磁椒，進(jìn)而分析這種思想如何應(yīng)用到Tomcat這個(gè)web容器中郊供，從源碼的角度對(duì) 違反ClassLoader雙親委派機(jī)制三部曲之首部——JDBC驅(qū)動(dòng)加載 中提出的Tomcat是如何完成多個(gè)web應(yīng)用之間相互隔離，又如何保證多個(gè)web應(yīng)用都能加載到基礎(chǔ)類庫(kù)的問(wèn)題進(jìn)行了解答削饵，我們按如下的思路布局整篇文章：
- 先給出Tomcat整體的類加載體系結(jié)構(gòu)
- 通過(guò)查看源碼驗(yàn)證該類加載體系的正確性
- 總結(jié)Tomcat如何設(shè)計(jì)保證多應(yīng)用隔離
另外本文是基于Tomcat7的源碼進(jìn)行分析的加叁，因此讀者最好先搭建一套基于Tomcat7的環(huán)境扮休，以便查閱源碼以及運(yùn)行調(diào)試锣枝，可以按照該文章的方式進(jìn)行搭建：Tomcat源碼導(dǎo)入Idea
Tomcat類加載體系結(jié)構(gòu)
Tomcat本身也是一個(gè)java項(xiàng)目厢拭，因此其也需要被JDK的類加載機(jī)制加載，也就必然存在引導(dǎo)類加載器惊橱、擴(kuò)展類加載器和應(yīng)用(系統(tǒng))類加載器蚪腐。Tomcat自身定義的類加載器主要由圖中下半部分組成，Common ClassLoader作為Catalina ClassLoader和Shared ClassLoader的parent税朴，而Shared ClassLoader又可能存在多個(gè)children類加載器WebApp ClassLoader，一個(gè)WebApp ClassLoader實(shí)際上就對(duì)應(yīng)一個(gè)Web應(yīng)用家制，那Web應(yīng)用就有可能存在Jsp頁(yè)面正林，這些Jsp頁(yè)面最終會(huì)轉(zhuǎn)成class類被加載，因此也需要一個(gè)Jsp的類加載器颤殴，就是圖中的JasperLoder
需要注意的是觅廓，在代碼層面Catalina ClassLoader、Shared ClassLoader涵但、Common ClassLoader對(duì)應(yīng)的實(shí)體類實(shí)際上都是URLClassLoader或者SecureClassLoader杈绸，一般我們只是根據(jù)加載內(nèi)容的不同和加載父子順序的關(guān)系，在邏輯上劃分為這三個(gè)類加載器矮瘟；而WebApp ClassLoader和JasperLoader都是存在對(duì)應(yīng)的類加載器類的
下面我們從源碼設(shè)計(jì)的角度驗(yàn)證圖中類加載器的設(shè)計(jì)
源碼分析Tomcat類加載機(jī)制
Tomcat的啟動(dòng)入口在Bootstrap.class中
其中初始化類加載器的流程在bootstrap.init();中瞳脓，如下“代碼清單1”
public void init()
throws Exception
{
// Set Catalina path
setCatalinaHome();
setCatalinaBase();
// (1) 初始化 classLoader
initClassLoaders();
Thread.currentThread().setContextClassLoader(catalinaLoader);
SecurityClassLoad.securityClassLoad(catalinaLoader);
// Load our startup class and call its process() method
if (log.isDebugEnabled())
log.debug("Loading startup class");
//加載 org.apache.catalina.startup.Catalina class
Class<?> startupClass =
catalinaLoader.loadClass
("org.apache.catalina.startup.Catalina");
// (2) 實(shí)例化 Catalina 實(shí)例
Object startupInstance = startupClass.newInstance();
// Set the shared extensions class loader
if (log.isDebugEnabled())
log.debug("Setting startup class properties");
String methodName = "setParentClassLoader";
Class<?> paramTypes[] = new Class[1];
paramTypes[0] = Class.forName("java.lang.ClassLoader");
Object paramValues[] = new Object[1];
paramValues[0] = sharedLoader;
Method method =
startupInstance.getClass().getMethod(methodName, paramTypes);
method.invoke(startupInstance, paramValues);
catalinaDaemon = startupInstance;
}
(1)處注釋的代碼主要進(jìn)行類加載的初始化以及形成類加載器的關(guān)系初始化，繼續(xù)跟進(jìn)
這里紅線處的代碼實(shí)際上創(chuàng)建了三個(gè)ClassLoader對(duì)象澈侠，其名稱和Tomcat類加載關(guān)系圖中的類加載器高度一致劫侧，那么我們猜測(cè)createClassLoader(String,ClassLoader)方法可能就是創(chuàng)建Tomcat自定義類加載器的方法之一，繼續(xù)往下看 “ 代碼清單2 ”
private ClassLoader createClassLoader(String name, ClassLoader parent)
throws Exception {
// (1) 根據(jù)名稱查找特定的配置
String value = CatalinaProperties.getProperty(name + ".loader");
if ((value == null) || (value.equals("")))
return parent;
value = replace(value);
List<Repository> repositories = new ArrayList<Repository>();
StringTokenizer tokenizer = new StringTokenizer(value, ",");
while (tokenizer.hasMoreElements()) {
String repository = tokenizer.nextToken().trim();
if (repository.length() == 0) {
continue;
}
// Check for a JAR URL repository
try {
@SuppressWarnings("unused")
URL url = new URL(repository);
repositories.add(
new Repository(repository, RepositoryType.URL));
continue;
} catch (MalformedURLException e) {
// Ignore
}
// Local repository
if (repository.endsWith("*.jar")) {
repository = repository.substring
(0, repository.length() - "*.jar".length());
repositories.add(
new Repository(repository, RepositoryType.GLOB));
} else if (repository.endsWith(".jar")) {
repositories.add(
new Repository(repository, RepositoryType.JAR));
} else {
repositories.add(
new Repository(repository, RepositoryType.DIR));
}
}
// (2) 類加載器工廠創(chuàng)建特定類加載器
return ClassLoaderFactory.createClassLoader(repositories, parent);
}
代碼清單中(1)處注釋是根據(jù)上圖中傳遞的“名稱”加上后綴.loader去某個(gè)配置文件加載文件哨啃，為了突出重點(diǎn)烧栋，這里直接給出結(jié)論，其加載的內(nèi)容為/org/apache/catalina/startup/catalina.properties拳球，比如要加載 common.loader對(duì)應(yīng)的value审姓，其在文件中的值為${catalina.base}/lib,${catalina.base}/lib/*.jar,${catalina.home}/lib,${catalina.home}/lib/*.jar，也就是說(shuō)Common ClassLoader要加載的路徑是這些祝峻，是Tomcat運(yùn)行要使用的公共組件魔吐，比如servlet-api.jar次坡、catalina.jar等；而我們發(fā)現(xiàn)當(dāng)要加載server.loader和shared.loader時(shí)画畅，其key在配置文件中的value為空砸琅，也就是說(shuō)，默認(rèn)情況下Catalina ClassLoader和Shared ClassLoader(Tomcat整體類加載體系結(jié)構(gòu)圖中紅色虛線內(nèi))都不存在轴踱，只有Common ClassLoader
方法中的第二個(gè)參數(shù)表示創(chuàng)建類加載器的父類加載器是哪個(gè)症脂，再看initClassLoaders()方法圖中代碼，在創(chuàng)建catalinaLoader和sharedLoader時(shí)淫僻，父類加載器傳入的實(shí)際上就是commonLoader诱篷，以此可以驗(yàn)證圖1中Catalina ClassLoader、Shared ClassLoader和Common ClassLoader的父子關(guān)系雳灵。而common ClassLoader的父類加載器參數(shù)傳遞的為null棕所，為什么null就會(huì)導(dǎo)致該類加載器的父類加載器為System ClassLoader呢？我們需要進(jìn)入代碼清單2中看注釋(2)處標(biāo)識(shí)的代碼 代碼清單3
public static ClassLoader createClassLoader(List<Repository> repositories,
final ClassLoader parent)
throws Exception {
if (log.isDebugEnabled())
log.debug("Creating new class loader");
// Construct the "class path" for this class loader
Set<URL> set = new LinkedHashSet<URL>();
// 加載指定路徑下的資源對(duì)象
if (repositories != null) {
for (Repository repository : repositories) {
if (repository.getType() == RepositoryType.URL) {
URL url = buildClassLoaderUrl(repository.getLocation());
if (log.isDebugEnabled())
log.debug(" Including URL " + url);
set.add(url);
} else if (repository.getType() == RepositoryType.DIR) {
File directory = new File(repository.getLocation());
directory = directory.getCanonicalFile();
if (!validateFile(directory, RepositoryType.DIR)) {
continue;
}
URL url = buildClassLoaderUrl(directory);
if (log.isDebugEnabled())
log.debug(" Including directory " + url);
set.add(url);
} else if (repository.getType() == RepositoryType.JAR) {
File file=new File(repository.getLocation());
file = file.getCanonicalFile();
if (!validateFile(file, RepositoryType.JAR)) {
continue;
}
URL url = buildClassLoaderUrl(file);
if (log.isDebugEnabled())
log.debug(" Including jar file " + url);
set.add(url);
} else if (repository.getType() == RepositoryType.GLOB) {
File directory=new File(repository.getLocation());
directory = directory.getCanonicalFile();
if (!validateFile(directory, RepositoryType.GLOB)) {
continue;
}
if (log.isDebugEnabled())
log.debug(" Including directory glob "
+ directory.getAbsolutePath());
String filenames[] = directory.list();
if (filenames == null) {
continue;
}
for (int j = 0; j < filenames.length; j++) {
String filename = filenames[j].toLowerCase(Locale.ENGLISH);
if (!filename.endsWith(".jar"))
continue;
File file = new File(directory, filenames[j]);
file = file.getCanonicalFile();
if (!validateFile(file, RepositoryType.JAR)) {
continue;
}
if (log.isDebugEnabled())
log.debug(" Including glob jar file "
+ file.getAbsolutePath());
URL url = buildClassLoaderUrl(file);
set.add(url);
}
}
}
}
// Construct the class loader itself
final URL[] array = set.toArray(new URL[set.size()]);
if (log.isDebugEnabled())
for (int i = 0; i < array.length; i++) {
log.debug(" location " + i + " is " + array[i]);
}
// 返回創(chuàng)建的類加載器
return AccessController.doPrivileged(
new PrivilegedAction<URLClassLoader>() {
@Override
public URLClassLoader run() {
if (parent == null)
return new URLClassLoader(array);
else
return new URLClassLoader(array, parent);
}
});
}
大塊的if中的代碼實(shí)際上是對(duì)資源進(jìn)行轉(zhuǎn)化加載的過(guò)程悯辙，而return部分才是返回類加載器的部分琳省，代碼根據(jù)是否有parent調(diào)用了URLClassLoader不同的構(gòu)造器，Common ClassLoader調(diào)用的是沒(méi)有parent的構(gòu)造器
按紅線所畫Common ClassLoader的parent實(shí)際上是JDK中sun.misc.Launcher.class類的loader成員變量躲撰，而在上一篇文章中已經(jīng)知道該loader的值就是應(yīng)用類加載器（系統(tǒng)類加載器）System ClassLoader针贬。至此Tomcat中類加載機(jī)制和JDK的類加載機(jī)制也建立上了聯(lián)系
現(xiàn)在Tomcat的類加載機(jī)制已完成了一大半，剩下用于加載每個(gè)web應(yīng)用的類加載器WebApp ClassLoader的分析拢蛋，這個(gè)時(shí)候需要重新回到代碼清單1中看注釋(2)以下的部分桦他，其主要做的事情是通過(guò)反射創(chuàng)建了org.apache.catalina.startup.Catalina類的實(shí)例，然后調(diào)用了簽名為void setParentClassLoader(ClassLoader parentClassLoader)的方法谆棱，并傳入了Shared ClassLoader快压，上面我們說(shuō)過(guò)默認(rèn)情況下Shared ClassLoader就是Common ClassLoader，因此其傳入的參數(shù)實(shí)際上是Common ClassLoader
我們思考既然有保存parent的方法垃瞧，必定使用時(shí)會(huì)調(diào)用獲得parent方法蔫劣，那么我們需要查看Catalina類中ClassLoader getParentClassLoader()方法的調(diào)用棧（層級(jí)關(guān)系比較復(fù)雜，要緊跟主線不要迷失）皆警，最終定位到StandardContext中的synchronized void startInternal() throws LifecycleException方法（其中涉及到Tomcat的各種組件關(guān)系拦宣，生命周期管理等內(nèi)容，將在下次分析Tomcat組件文章中詳細(xì)介紹）信姓，下面是只保留核心邏輯的startInternal()方法 代碼清單4
protected synchronized void startInternal() throws LifecycleException {
// 其他邏輯略......
// Add missing components as necessary
if (webappResources == null) { // (1) Required by Loader
if (log.isDebugEnabled())
log.debug("Configuring default Resources");
try {
String docBase = getDocBase();
if (docBase == null) {
setResources(new EmptyDirContext());
} else if (docBase.endsWith(".war")
&& !(new File(getBasePath())).isDirectory()) {
setResources(new WARDirContext());
} else {
setResources(new FileDirContext());
}
} catch (IllegalArgumentException e) {
log.error(sm.getString("standardContext.resourcesInit"), e);
ok = false;
}
}
if (ok) {
if (!resourcesStart()) {
throw new LifecycleException("Error in resourceStart()");
}
}
// (1) 為每一個(gè)web應(yīng)用創(chuàng)建一個(gè)WebappLoader
if (getLoader() == null) {
WebappLoader webappLoader = new WebappLoader(getParentClassLoader());
webappLoader.setDelegate(getDelegate());
setLoader(webappLoader);
}
// 其他邏輯略......
try {
if (ok) {
// (2) 調(diào)用WebappLoader的start
// Start our subordinate components, if any
if ((loader != null) && (loader instanceof Lifecycle))
((Lifecycle) loader).start();
}
// 其他邏輯省略......
} finally {
// Unbinding thread
unbindThread(oldCCL);
}
}
(1)處注釋下的代碼邏輯就是為每一個(gè)web應(yīng)用創(chuàng)建一個(gè)類加載器鸵隧，該類加載器的父類加載器就是通過(guò)getParentClassLoader()得到的Shared ClassLoader(Common ClassLoader)，(2)處代碼調(diào)用了WebappLoader的start方法意推，繼續(xù)跟進(jìn)
protected void startInternal() throws LifecycleException {
// 其他邏輯省略.....
try {
//創(chuàng)建類加載器關(guān)鍵方法
classLoader = createClassLoader();
classLoader.setResources(container.getResources());
classLoader.setDelegate(this.delegate);
classLoader.setSearchExternalFirst(searchExternalFirst);
if (container instanceof StandardContext) {
classLoader.setAntiJARLocking(
((StandardContext) container).getAntiJARLocking());
classLoader.setClearReferencesRmiTargets(
((StandardContext) container).getClearReferencesRmiTargets());
classLoader.setClearReferencesStatic(
((StandardContext) container).getClearReferencesStatic());
classLoader.setClearReferencesStopThreads(
((StandardContext) container).getClearReferencesStopThreads());
classLoader.setClearReferencesStopTimerThreads(
((StandardContext) container).getClearReferencesStopTimerThreads());
classLoader.setClearReferencesHttpClientKeepAliveThread(
((StandardContext) container).getClearReferencesHttpClientKeepAliveThread());
}
// 其他邏輯省略.....
}
由于Tomcat的設(shè)計(jì)豆瘫，WebappLoader的start方法實(shí)際上調(diào)用的是父類的模板，而模板中的startinternal方法由各個(gè)子類具體實(shí)現(xiàn)菊值，其中最關(guān)鍵的方法為createClassLoader()
上圖中的loadClass成員變量的值為org.apache.catalina.loader.WebappClassLoader外驱，所以育灸，實(shí)際上該類為每一個(gè)web應(yīng)用創(chuàng)建了一個(gè)WebappClassLoader的實(shí)例，該實(shí)例的parent就是Shared ClassLoader或者Common ClassLoader昵宇，至此WebApp ClassLoader在圖1中的位置也得以驗(yàn)證磅崭。
從理論上分析來(lái)看，由于類加載的“雙親委派”機(jī)制瓦哎，一個(gè)類加載器只能加載本加載器指定的目錄以及使用有“繼承”關(guān)系的父類加載器加載過(guò)的類砸喻，而Tomcat為每一個(gè)Web應(yīng)用創(chuàng)建了一個(gè)WebappClassLoader，不同的WebappClassLoader是同級(jí)關(guān)系蒋譬，不會(huì)存在交叉訪問(wèn)的問(wèn)題割岛，從而達(dá)到web應(yīng)用相互隔離的目的。
那Tomcat是否沒(méi)有"破壞"雙親委派機(jī)制呢犯助？我們通過(guò)查看WebappClassLoader及其父類WebappClassLoaderBase的loadClass()和findClass()分析一下Tomcat加載web應(yīng)用相關(guān)類的策略
public Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
synchronized (getClassLoadingLockInternal(name)) {
if (log.isDebugEnabled())
log.debug("loadClass(" + name + ", " + resolve + ")");
Class<?> clazz = null;
// Log access to stopped classloader
if (!started) {
try {
throw new IllegalStateException();
} catch (IllegalStateException e) {
log.info(sm.getString("webappClassLoader.stopped", name), e);
}
}
// (1)
// Check our previously loaded local class cache
clazz = findLoadedClass0(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Returning class from cache");
if (resolve)
resolveClass(clazz);
return (clazz);
}
// (2)
// Check our previously loaded class cache
clazz = findLoadedClass(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Returning class from cache");
if (resolve)
resolveClass(clazz);
return (clazz);
}
// (3)
// Try loading the class with the system class loader, to prevent
// the webapp from overriding J2SE classes
try {
clazz = j2seClassLoader.loadClass(name);
if (clazz != null) {
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
// Ignore
}
// Permission to access this class when using a SecurityManager
if (securityManager != null) {
int i = name.lastIndexOf('.');
if (i >= 0) {
try {
securityManager.checkPackageAccess(name.substring(0,i));
} catch (SecurityException se) {
String error = "Security Violation, attempt to use " +
"Restricted Class: " + name;
if (name.endsWith("BeanInfo")) {
// BZ 57906: suppress logging for calls from
// java.beans.Introspector.findExplicitBeanInfo()
log.debug(error, se);
} else {
log.info(error, se);
}
throw new ClassNotFoundException(error, se);
}
}
}
// (4)
boolean delegateLoad = delegate || filter(name);
// (5)
// Delegate to our parent if requested
if (delegateLoad) {
if (log.isDebugEnabled())
log.debug(" Delegating to parent classloader1 " + parent);
try {
clazz = Class.forName(name, false, parent);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Loading class from parent");
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
// Ignore
}
}
// (6)
// Search local repositories
if (log.isDebugEnabled())
log.debug(" Searching local repositories");
try {
clazz = findClass(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Loading class from local repository");
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
// Ignore
}
// Delegate to parent unconditionally
if (!delegateLoad) {
if (log.isDebugEnabled())
log.debug(" Delegating to parent classloader at end: " + parent);
try {
clazz = Class.forName(name, false, parent);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Loading class from parent");
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
// Ignore
}
}
}
throw new ClassNotFoundException(name);
}
我們首先定位到WebappClassLoaderBase的loadClass方法癣漆，(1)處首先看name對(duì)應(yīng)的類是否存在緩存中，緩存是一個(gè)ConcurrentHashMap<String, ResourceEntry>的集合剂买，如果沒(méi)有緩存執(zhí)行(2)處邏輯惠爽，從JVM中查找是否曾今加載過(guò)該類，(3)中的代碼保證自定義類不會(huì)覆蓋java基礎(chǔ)類庫(kù)中的類雷恃，(4)的邏輯就是是否進(jìn)行雙親委派的分叉口疆股，其中delegate默認(rèn)為false，那么就要看filter(String)方法倒槐，該方法的內(nèi)部實(shí)際上將待加載類的全路徑名稱和一個(gè)成員變量protected static final String[] packageTriggers中的類名進(jìn)行比較，如果待加載的類名和packageTriggers數(shù)組中的內(nèi)容前綴匹配附井，則需要委派父類加載讨越，即執(zhí)行(5)處代碼，否則執(zhí)行(6)永毅，調(diào)用重寫的findClass(String)方法加載該類
public Class<?> findClass(String name) throws ClassNotFoundException {
// 其他代碼略去.....
// Ask our superclass to locate this class, if possible
// (throws ClassNotFoundException if it is not found)
Class<?> clazz = null;
try {
if (log.isTraceEnabled())
log.trace(" findClassInternal(" + name + ")");
// (1)
if (hasExternalRepositories && searchExternalFirst) {
try {
clazz = super.findClass(name);
} catch(ClassNotFoundException cnfe) {
// Ignore - will search internal repositories next
} catch(AccessControlException ace) {
log.warn("WebappClassLoaderBase.findClassInternal(" + name
+ ") security exception: " + ace.getMessage(), ace);
throw new ClassNotFoundException(name, ace);
} catch (RuntimeException e) {
if (log.isTraceEnabled())
log.trace(" -->RuntimeException Rethrown", e);
throw e;
}
}
// (2)
if ((clazz == null)) {
try {
clazz = findClassInternal(name);
} catch(ClassNotFoundException cnfe) {
if (!hasExternalRepositories || searchExternalFirst) {
throw cnfe;
}
} catch(AccessControlException ace) {
log.warn("WebappClassLoaderBase.findClassInternal(" + name
+ ") security exception: " + ace.getMessage(), ace);
throw new ClassNotFoundException(name, ace);
} catch (RuntimeException e) {
if (log.isTraceEnabled())
log.trace(" -->RuntimeException Rethrown", e);
throw e;
}
}
//其他代碼略去........
return (clazz);
}
(1)處由于hasExternalRepositories和searchExternalFirst默認(rèn)為false把跨，因此執(zhí)行(2)處邏輯，調(diào)用findClassInternal(String)方法
其主要的思想是根據(jù)待加載類的全路徑讀取該類的二進(jìn)制數(shù)據(jù)沼死，進(jìn)而進(jìn)行類的預(yù)定義着逐、class source的解析等，將該類加載到JVM中
綜上所述意蛀，我認(rèn)為Tomcat的類加載機(jī)制不能算完全“正統(tǒng)”的雙親委派耸别，WebappClassLoader內(nèi)部重寫了loadClass和findClass方法，實(shí)現(xiàn)了繞過(guò)“雙親委派”直接加載web應(yīng)用內(nèi)部的資源县钥，當(dāng)然可以通過(guò)在Context.xml文件中加上<Loader delegate = "true">開啟正統(tǒng)的“雙親委派”加載機(jī)制
