TOMCAT学习之类加载器
Tomcat引入自定义类加载器是基于这几点考虑：1、servlet不能完全信任它正在运行的servlet类，所以不能简单的使用系统类加载器；2、为了实现类发生变化时自动重载功能。
Tomcat在启动引导类Bootstrap中对类加载器进行了初始化，以方便在后面的运行过程中加载所需要的类。接下来我们分析一下Tomcat如何创建类加载器。
1、加载conf目录下属性配置文件catalina.properties，分别获取common.loader、server.loader、shared.loader等属性对应的值，并进行处理，具体见下面代码：
代码：
private ClassLoader createClassLoader(String name, ClassLoader parent)
throws Exception {
//加载conf目录下属性配置文件catalina.properties，
//分别获取common.loader、server.loader、shared.loader等属性
String value = CatalinaProperties.getProperty(name + ".loader");
if ((value == null) || (value.equals("")))
return parent;
//将${catalina.base}/${catalina.home}进行替换
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));
}
}
//用类加载器工厂创建类加载器
ClassLoader classLoader = ClassLoaderFactory.createClassLoader
(repositories, parent);
// Retrieving MBean server
//下面这部分代码，大家可以参考JMX相关的知识
MBeanServer mBeanServer = null;
if (MBeanServerFactory.findMBeanServer(null).size() > 0) {
mBeanServer = MBeanServerFactory.findMBeanServer(null).get(0);
} else {
mBeanServer = ManagementFactory.getPlatformMBeanServer();
}
// Register the server classloader
ObjectName objectName =
new ObjectName("Catalina:type=ServerClassLoader,name=" + name);
mBeanServer.registerMBean(classLoader, objectName);
return classLoader;
}
2、接下来我们看一下ClassLoaderFactory中是如何创建类加载器的：
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>();
/**
* 对仓库中的资源类型进行判断，
* 1、 如果为URL类型就加入set集合，
* 2、如果为其他类型，则会判断它是否为文件，如果是则进行转换并加入set集合，否则跳过；
* 最后创建 StandardClassLoader 对象并返回
*/
if (repositories != null) {
for (Repository repository : repositories) {
if (repository.getType() == RepositoryType.URL) {
URL url = new URL(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 = directory.toURI().toURL();
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 = file.toURI().toURL();
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();
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 = file.toURI().toURL();
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]);
}
//这里AccessController.doPrivileged意思是这个是特别的,不用做权限检查
return AccessController.doPrivileged(
new PrivilegedAction<StandardClassLoader>() {
@Override
public StandardClassLoader run() {
if (parent == null)
return new StandardClassLoader(array);
else
return new StandardClassLoader(array, parent);
}
});
}
3、得到classloader之后，便可以加载需要用到的类了：
例1：SecurityClassLoad.securityClassLoad(catalinaLoader);
例2：Class startupClass = catalinaLoader.loadClass ("org.apache.catalina.startup.Catalina");