类装载器
JDK中提供了3种不同的类加载器:启动类装载器,扩展类装载器和系统类装载器。引导类装载器,用于引导启动JAVA虚拟机,当执行一个JAVA程序时,就会启动引导类装载器,它是使用本地代码来实现的,会装载%JAVA_HOME%\\jre\lib\rt.jar,它是所有类装载器类的父装载器。扩展类装载器负责载入标准扩展目录中的类,其搜索路径是%JAVA_HOME%\jre\lib\ext,只需要将打包好的jar文件放入这个目录就可以了,给开发提供了很大的便利性。系统类装载器是默认的装载器,其搜索路径是classpath。
JVM到底使用的是哪一个类装载器,取决于类装载器的代理模式。每当需要装载一个类的时候,会首先调用系统类装载器,但是系统类装载器并不会立即装载,而是将其交给父装载器:扩展类装载器,扩展类装载器其将交给引导类装载器,引导类装载器没有父装载器,它会尝试装载这个类,如果找不到这个类,会交给扩展类装载器,如果扩展类装载器还是没有找到,会交给系统类装载器,如果系统类装载器还是没有找到这个类,则会抛出java.lang.ClassNotFoundException异常。代理模式主要是为了解决类装载的安全问题。例如:对于自定类的java.lang.Object类,永远得不到装载,除非,rt.jar中确实没有这个类。
tomcat也提供了几种不同的类装载器用于加载不同位置的jar包和class文件,特别是Context容器需要有一个单独的类装载器,因为不同应用可能有相同的类,如果用同一个类装载器去装载,就不知道该加载哪个应用里面的类了。这些类装载器之间的关系如下图所示:
系统类装载器
tomcat的系统类装载器和JDK的系统类装载器有点不同的地方是搜索路径并不相同,在catalina.bat中做了如下修改:
rem Add on extra jar file to CLASSPATH
rem Note that there are no quotes as we do not want to introduce random
rem quotes into the CLASSPATH
if "%CLASSPATH%" == "" goto emptyClasspath
set "CLASSPATH=%CLASSPATH%;"
:emptyClasspath
set "CLASSPATH=%CLASSPATH%%CATALINA_HOME%\bin\bootstrap.jar"
rem Add tomcat-juli.jar to classpath
rem tomcat-juli.jar can be over-ridden per instance
if not exist "%CATALINA_BASE%\bin\tomcat-juli.jar" goto juliClasspathHome
set "CLASSPATH=%CLASSPATH%;%CATALINA_BASE%\bin\tomcat-juli.jar"
先将classpath清空,因为classpath中可能有tomcat启动相关的类会影响tomcat的正常启动。然后将bootstrap.jar和tomcat-juli.jar加入classpath中,在catalina.bat中调用了Bootstrap类的main方法,这里系统类装载器会装载Bootstrap类,Bootstrap类用到的Catalina类也是由系统类装载器装载的。public void init() throws Exception{
// Set Catalina path
setCatalinaHome();
setCatalinaBase();
initClassLoaders();
Thread.currentThread().setContextClassLoader(catalinaLoader);
SecurityClassLoad.securityClassLoad(catalinaLoader);
...
}
private void initClassLoaders() {
try {
commonLoader = createClassLoader("common", null);
if( commonLoader == null ) {
// no config file, default to this loader - we might be in a 'single' env.
commonLoader=this.getClass().getClassLoader();
}
catalinaLoader = createClassLoader("server", commonLoader);
sharedLoader = createClassLoader("shared", commonLoader);
} catch (Throwable t) {
handleThrowable(t);
log.error("Class loader creation threw exception", t);
System.exit(1);
}
}
Common Class Loader
private ClassLoader createClassLoader(String name, ClassLoader parent)throws Exception {
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));
}
}
ClassLoader classLoader = ClassLoaderFactory.createClassLoader(repositories, parent);
...
return classLoader;
}
(1)、从bootstrap.jar包可以找到catalina.properties文件:common.loader=${catalina.home}/lib,${catalina.home}/lib/*.jar,很明显这个类装载器搜索路径就是${catalina.home}/lib和${catalina.home}/lib/*.jar,之所以叫common class loader,是因为它加载每个应用要用到的公共jar包和class文件;Catalina Class Loader
common.loader=${catalina.base}/lib,${catalina.base}/lib/*.jar,${catalina.home}/lib,${catalina.home}/lib/*.jar
server.loader=
shared.loader=
common class loader是以common class loader为父装载器的,因此其搜索路径和common class loader一样。catalina class loader创建好后,在init方法中随即调用了Thread.currentThread().setContextClassLoader(catalinaLoader);将其设置为当前线程的类装载器Shared Class Loader
Webapp Class Loader
tomcat的一个service除了容器和连接器外还有很多组件,比如sessionManager,logger,loader等,这个类装载器是以组件的形式附着在每个容器上的,Engine和Host的这两个容器的loader组件为null,context里面是有值的,看看context的startInternal方法:
protected synchronized void startInternal() throws LifecycleException {
...
if (getLoader() == null) {
WebappLoader webappLoader = new WebappLoader(getParentClassLoader());
webappLoader.setDelegate(getDelegate());
setLoader(webappLoader);
}
...
// Binding thread
ClassLoader oldCCL = bindThread();
try {
if (ok) {
// Start our subordinate components, if any
if ((loader != null) && (loader instanceof Lifecycle))
((Lifecycle) loader).start();
// since the loader just started, the webapp classloader is now
// created.
// By calling unbindThread and bindThread in a row, we setup the
// current Thread CCL to be the webapp classloader
unbindThread(oldCCL);
oldCCL = bindThread();
}
...
} finally {
// Unbinding thread
unbindThread(oldCCL);
}
}
很明显,context在启动的时候创建了一个loader组件,webapploader正是loader的实现类,这个类并不是最终的类装载器,在这个类里面有一个webappclassloader类型的字段叫classloader,这个classloader的创建是在loader组件的start方法中完成的 protected void startInternal() throws LifecycleException {
...
// Construct a class loader based on our current repositories list
try {
classLoader = createClassLoader();
classLoader.setResources(container.getResources());
classLoader.setDelegate(this.delegate);
classLoader.setSearchExternalFirst(searchExternalFirst);
if (container instanceof StandardContext) {
classLoader.setAntiJARLocking(
((StandardContext) container).getAntiJARLocking());
classLoader.setClearReferencesStatic(
((StandardContext) container).getClearReferencesStatic());
classLoader.setClearReferencesStopThreads(
((StandardContext) container).getClearReferencesStopThreads());
classLoader.setClearReferencesStopTimerThreads(
((StandardContext) container).getClearReferencesStopTimerThreads());
classLoader.setClearReferencesHttpClientKeepAliveThread(
((StandardContext) container).getClearReferencesHttpClientKeepAliveThread());
}
for (int i = 0; i < repositories.length; i++) {
classLoader.addRepository(repositories[i]);
}
// Configure our repositories
setRepositories();
setClassPath();
setPermissions();
((Lifecycle) classLoader).start();
...
} catch (Throwable t) {
...
}
...
}
(1)、在createClassLoader方法中通过反射实例化了org.apache.catalina.loader.WebappClassLoader这个类,并调用了它的setParentClassLoader设置其父装载器为standardClassLoader private WebappClassLoader createClassLoader()
throws Exception {
Class<?> clazz = Class.forName(loaderClass);
WebappClassLoader classLoader = null;
if (parentClassLoader == null) {
parentClassLoader = container.getParentClassLoader();
}
Class<?>[] argTypes = { ClassLoader.class };
Object[] args = { parentClassLoader };
Constructor<?> constr = clazz.getConstructor(argTypes);
classLoader = (WebappClassLoader) constr.newInstance(args);
return classLoader;
}
(2)、为webappclassloader添加仓库(仓库表示类装载器会在哪些路径搜索类)private void setRepositories() throws IOException {
...
// Setting up the class repository (/WEB-INF/classes), if it exists
String classesPath = "/WEB-INF/classes";
...
// Adding the repository to the class loader
classLoader.addRepository(classesPath + "/", classRepository);
// Setting up the JAR repository (/WEB-INF/lib), if it exists
String libPath = "/WEB-INF/lib";
...
// Looking up directory /WEB-INF/lib in the context
NamingEnumeration<NameClassPair> enumeration = libDir.list("");
while (enumeration.hasMoreElements()) {
NameClassPair ncPair = enumeration.nextElement();
String filename = libPath + "/" + ncPair.getName();
if (!filename.endsWith(".jar"))
continue;
...
try {
JarFile jarFile = new JarFile(destFile);
classLoader.addJar(filename, jarFile, destFile);
} catch (Exception ex) {
...
}
...
}
}
(3)、为类装载器设置权限,这里Globals.IS_SECURITY_ENABLED值为false,表示安全机制未打开,直接返回 protected static final String[] triggers = {
"javax.servlet.Servlet", "javax.el.Expression" // Servlet API
};
WebappClassLoader装载类
public synchronized Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
...
// 先检查本地缓存
clazz = findLoadedClass0(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Returning class from cache");
if (resolve)
resolveClass(clazz);
return (clazz);
}
// 如果本地缓存没有,则检查上一级缓存
clazz = findLoadedClass(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Returning class from cache");
if (resolve)
resolveClass(clazz);
return (clazz);
}
// 如果两个缓存都没有,则使用系统的类装载器进行装载,防止Web应用程序中的类覆盖J2EE的类
try {
clazz = system.loadClass(name);
if (clazz != null) {
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
// Ignore
}
// 如果启用了SecurityManager,则检查此类是否允许被载入,如果不允许,则抛出异常
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;
log.info(error, se);
throw new ClassNotFoundException(error, se);
}
}
}
boolean delegateLoad = delegate || filter(name);
// 若打开了delegateLoad标志位,调用父装载器来加载。如果父装载器为null,使用系统类装载器装载
if (delegateLoad) {
if (log.isDebugEnabled())
log.debug(" Delegating to parent classloader1 " + parent);
ClassLoader loader = parent;
if (loader == null)
loader = system;
try {
clazz = Class.forName(name, false, loader);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Loading class from parent");
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
// Ignore
}
}
// 从本地仓库中载入相关类
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
}
// 若当前仓库中没有需要的类,且delegateLoad标志位关闭,则使用父装载器。若父装载器为null,使用系统类装载器来装载
if (!delegateLoad) {
if (log.isDebugEnabled())
log.debug(" Delegating to parent classloader at end: " + parent);
ClassLoader loader = parent;
if (loader == null)
loader = system;
try {
clazz = Class.forName(name, false, loader);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Loading class from parent");
if (resolve)
resolveClass(clazz);
return (clazz);
}
} catch (ClassNotFoundException e) {
// Ignore
}
}
//仍未找到,抛出异常
throw new ClassNotFoundException(name);
}
整个思路是:先到缓存中获取,如果缓存中有直接返回,否则根据delegateLoad采取不同的加载方式。如果未启用这个标志:先本地仓库加载再父装载器或者系统类装载器装载;如果启用了这个标志:直接由父装载器或者系统类装载器装载。