上文：tomcat热加载、热部署-源码解析
背景
继上文，那么你可能跟我开始一样，tomcat的类加载与我们的java有什么区别？是一样的还是有哪些区别？其次tomcat项目怎么隔离？其三tomat如何打破双亲委派机制？
相关基础
java类如何加载的？
这个问题建议阅读我以往的文章，有这个基础会更好了解如下。文章：
类的加载时机
jvm的类加载器(classloader)及类的加载过程
以前画的图，比较丑，别介意哈~
打破双亲委派机制
那么如何打破双亲委派机制，可以参考另一个文章：如何打破双亲委派机制？
看到这里你很明显清楚的知道tomcat其实就基于jdk的类加载机制进行重写Classloader，然后通过classloader进行重写findClass实现的。
tomcat为什么要打破双亲委派机制？
项目之间互相隔离：不同的项目，相同的路径起到互相隔离，方便多部署项目，保证项目之间互不影响（由于一个tomcat可能需要部署多套系统需要）;
共享java类库：不同的项目之间可以共享java的类库，不需要重装安装；
支持热部署：新项目加载或部署不会影响正在运行的项目;
tomcat打破双亲委派机制的实现源码
代码位置：org.apache.catalina.loader.WebappClassLoaderBase#loadClass(java.lang.String, boolean)
代码语言：javascript复制@Override
public Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
//同步锁 这里是调用ClassLoader的方法获取
synchronized (getClassLoadingLock(name)) {
if (log.isDebugEnabled()) {
log.debug("loadClass(" name ", " resolve ")");
}
Class<?> clazz = null;
// Log access to stopped class loader 检查当前的classloader是事已经停止，没有就跑出异常
checkStateForClassLoading(name);
// (0) Check our previously loaded local class cache
// 本地检查缓存中是否存在加载过了。这是指的是webappcloassloader
clazz = findLoadedClass0(name);
if (clazz != null) {
if (log.isDebugEnabled()) {
log.debug(" Returning class from cache");
}
if (resolve) {
resolveClass(clazz);
}
return clazz;
}
// (0.1) Check our previously loaded class cache
//从系统类加载器里面判断是否加载过了，如果是则进行返回 SystemClassLoader
clazz = findLoadedClass(name);
if (clazz != null) {
if (log.isDebugEnabled()) {
log.debug(" Returning class from cache");
}
//如果为直，调用classloader本地方法进行加载类；
if (resolve) {
resolveClass(clazz);
}
return clazz;
}
// (0.2) Try loading the class with the bootstrap class loader, to prevent
// the webapp from overriding Java SE classes. This implements
// SRV.10.7.2
//用于防止系统的类覆盖到jdk默认的lib
String resourceName = binaryNameToPath(name, false);
//通过ExtClassloader 查看是否被加载过了
ClassLoader javaseLoader = getJavaseClassLoader();
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;
}
//默认返回是成功，因为检查一般是通过，不通过会抛出异常
if (tryLoadingFromJavaseLoader) {
try {
//通过Classloader进行加载
clazz = javaseLoader.loadClass(name);
//不为空，进行解析并加载到缓存中后返回
if (clazz != null) {
if (resolve) {
resolveClass(clazz);
}
return clazz;
}
} catch (ClassNotFoundException e) {
// Ignore
}
}
// (0.5) Permission to access this class when using a SecurityManager
//安全检查 主要指包路劲
if (securityManager != null) {
int i = name.lastIndexOf('.');
if (i >= 0) {
try {
securityManager.checkPackageAccess(name.substring(0,i));
} catch (SecurityException se) {
String error = sm.getString("webappClassLoader.restrictedPackage", name);
log.info(error, se);
throw new ClassNotFoundException(error, se);
}
}
}
//判断是否使用委托方式（父类加载）
boolean delegateLoad = delegate || filter(name, true);
// (1) Delegate to our parent if requested
//delegateLoad用于判断是否委托加载
if (delegateLoad) {
if (log.isDebugEnabled()) {
log.debug(" Delegating to parent classloader1 " parent);
}
try {
//通过父类加载
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 {
//这里是核心的重写方法，因为这个是重写了classloader进行实现的。（webapp）
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
//不是委托加载，通过父类进行加载
if (!delegateLoad) {
if (log.isDebugEnabled()) {
log.debug(" Delegating to parent classloader at end: " parent);
}
try {
//通过父类加载
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
}
}
}
//都打不到抛出找不到类异常
throw new ClassNotFoundException(name);
}最后
通过了解java的类加载机制再来学习tomcat的类加载机制，特别是阅读过源码后你会发现非常简单，只是针对Classloader进行重新，根据自已的需求进行判断路劲是通过双亲委派机制进行加载，还是通过自定类加载器进行加载，所以这块建议学习的同学通过源码来学习会快很多，当然在学习之前特别建议把相关的基础一定要扎实。
参考文章：
https://www.jianshu.com/p/9b2d43c9a09a
https://blog.51cto.com/tiantianzaixian/5621528
视频：https://www.bilibili.com/video/BV1o34y197fz?p=8&vd_source=7d0e42b081e08cb3cefaea55cc1fa8b7
