Order of Tomcat Classloaders: common, shared, and server

Question

The Tomcat Class Loader HOW-TO documentation describes 4 different class loaders:

1. Bootstrap
2. System
3. Common
4. Webapp

In the d

Answer 1

I recently ran into this issue as well and here is what I found, (This is all from Tomcat 7 trunk)

If left as blank, the "common" loader will be used as Catalina's "shared"/"server" loader.

Here is the relevant source,

89      private void initClassLoaders() {
90          try {
91              commonLoader = createClassLoader("common", null);
92              if( commonLoader == null ) {
93                  // no config file, default to this loader - we might be in a 'single' env.
94                  commonLoader=this.getClass().getClassLoader();
95              }
96              catalinaLoader = createClassLoader("server", commonLoader);
97              sharedLoader = createClassLoader("shared", commonLoader);
98          } catch (Throwable t) {
99              handleThrowable(t);
100             log.error("Class loader creation threw exception", t);
101             System.exit(1);
102         }
103     }

106     private ClassLoader createClassLoader(String name, ClassLoader parent)
107         throws Exception {
108 
109         String value = CatalinaProperties.getProperty(name + ".loader");
110         if ((value == null) || (value.equals("")))
111             return parent;

So if nothing is defined, they fallback to using the common.loader entries.

---

As to the order that they are loaded in, here is the source for loading them in, from source

229         Thread.currentThread().setContextClassLoader(catalinaLoader);
230 
231         SecurityClassLoad.securityClassLoad(catalinaLoader);
232 
233         // Load our startup class and call its process() method
234         if (log.isDebugEnabled())
235             log.debug("Loading startup class");
236         Class startupClass =
237             catalinaLoader.loadClass
238             ("org.apache.catalina.startup.Catalina");
239         Object startupInstance = startupClass.newInstance();
240 
241         // Set the shared extensions class loader
242         if (log.isDebugEnabled())
243             log.debug("Setting startup class properties");
244         String methodName = "setParentClassLoader";
245         Class paramTypes[] = new Class[1];
246         paramTypes[0] = Class.forName("java.lang.ClassLoader");
247         Object paramValues[] = new Object[1];
248         paramValues[0] = sharedLoader;
249         Method method =
250             startupInstance.getClass().getMethod(methodName, paramTypes);
251         method.invoke(startupInstance, paramValues);

Line 229 sets the common.loader classLoader, then line 251 sets the shared.loader classloader is set as Catalinas parent class loader.