类加载机制是什么
- 概论
- .java文件通过编译以后，读取到jvm的方法区中，然后类加载器的加载、验证（class特定的格式）、准备（分配内存）、解析（将字节码的符号引用改成直接引用）、初始化（对象的初始化）过程
- 几种常见的类加载
- bootstrap class loader（顶级class loader，无父类加载器）
- 主要加载java的核心类库，rt,jar,resource.jar;可以通过XbootclassPath指定核心类库的路径
- extension class loader （父类加载器为boot strap classloader）
- 主要加载java核心类库下面的lib/ext ，这个是一个java类，会继承URLClassloader
- system classloader(appliction classloader ，父类的加载器（extension classloader）)
- 主要是加载应用下的java库；
- 自定义的classloader（父类加载为system class loader ）
- 继承抽象类classloader
- 自定义loadClass,findClass
- bootstrap class loader（顶级class loader，无父类加载器）
下面是java 8 中ClassLoad.loadclass()源码，如果开发人员需要自定义的加载器需要重新注意重新下面的loadClass和find classde 方法，从而可以打破双亲的委派机制
/**
* 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) {
//是一个递归调用，直至没有父类的类加载器
c = parent.loadClass(name, false);
} else {
//加载java核心类库的加载器
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();
//在指定的路径下获取到该类
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;
}
}
URLClassloader中的findclass方法
/**
* Finds and loads the class with the specified name from the URL search
* path. Any URLs referring to JAR files are loaded and opened as needed
* until the class is found.
*
* @param name the name of the class
* @return the resulting class
* @exception ClassNotFoundException if the class could not be found,
* or if the loader is closed.
* @exception NullPointerException if {@code name} is {@code null}.
*/
protected Class<?> findClass(final String name)
throws ClassNotFoundException
{
final Class<?> result;
try {
result = AccessController.doPrivileged(
new PrivilegedExceptionAction<Class<?>>() {
public Class<?> run() throws ClassNotFoundException {
//拼接成指定前缀的.class文件
String path = name.replace('.', '/').concat(".class");
//从classpath获取到.class文件
Resource res = ucp.getResource(path, false);
if (res != null) {
try {
//从路径中读取class文件字节流，并定义成类；todo bytebuffer
return defineClass(name, res);
} catch (IOException e) {
throw new ClassNotFoundException(name, e);
}
} else {
return null;
}
}
}, acc);
} catch (java.security.PrivilegedActionException pae) {
throw (ClassNotFoundException) pae.getException();
}
if (result == null) {
throw new ClassNotFoundException(name);
}
return result;
}
tomcat中的类加载机制是什么
上面通过源码，我们很清楚的了解到java默认的加载方式，都会去加载父类的加载，如何tomcat 按照这种方式加载，势必导致webapps中不同的context的的不同版本的jar，不能被加载到，tomcat是如何打破双亲委派机制的呢？
隔离不同应用：部署在同一个Tomcat中的不同应用A和B，例如A用了Spring2.5。B用了Spring3.5，那么这两个应用如果使用的是同一个类加载器，那么Web应用就会因为jar包覆盖而无法启动。
灵活性：Web应用之间的类加载器相互独立，那么就可以根据修改不同的文件重建不同的类加载器替换原来的。从而不影响其他应用。
性能：如果在一个Tomcat部署多个应用，多个应用中都有相同的类库依赖。那么可以把这相同的类库让Common类加载器进行加载。
tomcat 类加载器
通过tomcat启动类，bootstrap类中，有三个classloader，分别如下
ClassLoader commonLoader = null;//基础的classloader
ClassLoader catalinaLoader = null;//Catalina classloader
ClassLoader sharedLoader = null;//公共的classloader
通过源码发现tomcat 有三种类型的类加载器；分别为commonLoader,catalinaLoader,shareoader;
tomcat是如何打破的
通过自定义加载器的过程，我们知道，实现自定义的classloader，需要重新loadClass以及findClass，我们先看
webappsClassLoaderBase的下面两个方法；
/**
* Load the class with the specified name, searching using the following
* algorithm until it finds and returns the class. If the class cannot
* be found, returns <code>ClassNotFoundException</code>.
* <ul>
* <li>Call <code>findLoadedClass(String)</code> to check if the
* class has already been loaded. If it has, the same
* <code>Class</code> object is returned.</li>
* <li>If the <code>delegate</code> property is set to <code>true</code>,
* call the <code>loadClass()</code> method of the parent class
* loader, if any.</li>
* <li>Call <code>findClass()</code> to find this class in our locally
* defined repositories.</li>
* <li>Call the <code>loadClass()</code> method of our parent
* class loader, if any.</li>
* </ul>
* If the class was found using the above steps, and the
* <code>resolve</code> flag is <code>true</code>, this method will then
* call <code>resolveClass(Class)</code> on the resulting Class object.
*
* @param name The binary name of the class to be loaded
* @param resolve If <code>true</code> then resolve the class
*
* @exception ClassNotFoundException if the class was not found
*/
@Override
public Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
synchronized (getClassLoadingLock(name)) {
if (log.isDebugEnabled())
log.debug("loadClass(" + name + ", " + resolve + ")");
Class<?> clazz = null;
// Log access to stopped class loader
checkStateForClassLoading(name);
// (0) Check our previously loaded local class cache 本地缓存
clazz = findLoadedClass0(name);
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Returning class from cache");
if (resolve)
resolveClass(clazz);//链接classloader todo
return clazz;
}
// (0.1) Check our previously loaded class cache
clazz = findLoadedClass(name);//校验jvm 的appclassloader的缓存中是否存在
if (clazz != null) {
if (log.isDebugEnabled())
log.debug(" Returning class from cache");
if (resolve)
resolveClass(clazz);
return clazz;
}
// (0.2) Try loading the class with the system class loader, to prevent
// the webapp from overriding Java SE classes. This implements
// SRV.10.7.2
String resourceName = binaryNameToPath(name, false);
ClassLoader javaseLoader = getJavaseClassLoader();//系统的bootstrap classloader 的classloader
boolean tryLoadingFromJavaseLoader;
try {
// Use getResource as it won't trigger an expensive
// ClassNotFoundException if the resource is not available from
// the Java SE class loader. However (see
// https://bz.apache.org/bugzilla/show_bug.cgi?id=58125 for
// details) when running under a security manager in rare cases
// this call may trigger a ClassCircularityError.
// See https://bz.apache.org/bugzilla/show_bug.cgi?id=61424 for
// details of how this may trigger a StackOverflowError
// Given these reported errors, catch Throwable to ensure any
// other edge cases are also caught
URL url;
if (securityManager != null) {
PrivilegedAction<URL> dp = new PrivilegedJavaseGetResource(resourceName);
url = AccessController.doPrivileged(dp);
} else {
url = javaseLoader.getResource(resourceName);
}
tryLoadingFromJavaseLoader = (url != null);
} catch (Throwable t) {
// Swallow all exceptions apart from those that must be re-thrown
ExceptionUtils.handleThrowable(t);
// The getResource() trick won't work for this class. We have to
// try loading it directly and accept that we might get a
// ClassNotFoundException.
tryLoadingFromJavaseLoader = true;
}
if (tryLoadingFromJavaseLoader) {
try {
//利用javaser的加载方式 ，即双亲委派的模式，核心类库
clazz = javaseLoader.loadClass(name);
if (clazz != null) {
if (resolve)
resolveClass(clazz);
return clazz;
}
} catch (ClassNotFoundException e) {
// Ignore
}
}
// (0.5) 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;
log.info(error, se);
throw new ClassNotFoundException(error, se);
}
}
}
//是否使用委托方式,即利用父类加载器加载该类的方式
boolean delegateLoad = delegate || filter(name, true);
// (1) 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
}
}
// (2) Search local repositories
if (log.isDebugEnabled())
log.debug(" Searching local repositories");
try {
//不使用父类加载器的方式，直接重写findclass，
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
}
// (3) 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);
}
protected void checkStateForClassLoading(String className) throws ClassNotFoundException {
// It is not permitted to load new classes once the web application has
// been stopped.
try {
checkStateForResourceLoading(className);
} catch (IllegalStateException ise) {
throw new ClassNotFoundException(ise.getMessage(), ise);
}
}
@Override
public Class<?> findClass(String name) throws ClassNotFoundException {
if (log.isDebugEnabled())
log.debug(" findClass(" + name + ")");
checkStateForClassLoading(name);
// (1) Permission to define this class when using a SecurityManager
if (securityManager != null) {
int i = name.lastIndexOf('.');
if (i >= 0) {
try {
if (log.isTraceEnabled())
log.trace(" securityManager.checkPackageDefinition");
securityManager.checkPackageDefinition(name.substring(0,i));
} catch (Exception se) {
if (log.isTraceEnabled())
log.trace(" -->Exception-->ClassNotFoundException", se);
throw new ClassNotFoundException(name, se);
}
}
}
// 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 + ")");
try {
if (securityManager != null) {
PrivilegedAction<Class<?>> dp =
new PrivilegedFindClassByName(name);
clazz = AccessController.doPrivileged(dp);
} else {
//从本地的具体类名查找 内部方式 ；//webapps lib
clazz = findClassInternal(name);
}
} catch(AccessControlException ace) {
log.warn("WebappClassLoader.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;
}
if ((clazz == null) && hasExternalRepositories) {
try {
//调用父类的加载方式
clazz = super.findClass(name);
} catch(AccessControlException ace) {
log.warn("WebappClassLoader.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;
}
}
if (clazz == null) {
if (log.isDebugEnabled())
log.debug(" --> Returning ClassNotFoundException");
throw new ClassNotFoundException(name);
}
} catch (ClassNotFoundException e) {
if (log.isTraceEnabled())
log.trace(" --> Passing on ClassNotFoundException");
throw e;
}
// Return the class we have located
if (log.isTraceEnabled())
log.debug(" Returning class " + clazz);
if (log.isTraceEnabled()) {
ClassLoader cl;
if (Globals.IS_SECURITY_ENABLED){
cl = AccessController.doPrivileged(
new PrivilegedGetClassLoader(clazz));
} else {
//如果父类再加载不到的化，
cl = clazz.getClassLoader();
}
log.debug(" Loaded by " + cl.toString());
}
return clazz;
}
Web应用默认的类加载顺序是（打破了双亲委派规则）：
- 先从JVM的BootStrapClassLoader中加载。
- 加载Web应用下
/WEB-INF/classes中的类。 - 加载Web应用下
/WEB-INF/lib/*.jap中的jar包中的类。 - 加载上面定义的System路径下面的类。
- 加载上面定义的Common路径下面的类。
如果在配置文件中配置了``，那么就是遵循双亲委派规则，加载顺序如下：
- 先从JVM的BootStrapClassLoader中加载。
- 加载上面定义的System路径下面的类。
- 加载上面定义的Common路径下面的类。
- 加载Web应用下
/WEB-INF/classes中的类。 - 加载Web应用下
/WEB-INF/lib/*.jap中的jar包中的类。
参考文献
