tomcat 7 源码分析-2 类加载ClassLoader

tomcat在启动的时候使用了三个类加载器

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) {
            log.error("Class loader creation threw exception", t);
            System.exit(1);
        }
    }

 三个类加载器加载了catalina.properties中的jar

common.loader=${catalina.base}/lib,${catalina.base}/lib/*.jar,${catalina.home}/lib,${catalina.home}/lib/*.jar
server.loader=
shared.loader=

 由此可见三个类加载器加载的jar是一样的。为什么要用三个类加载器，从名称看应该是有分别的作用考虑，负载的考虑也许吧。

此外    private ClassLoader createClassLoader(String name, ClassLoader parent)中调用        ClassLoader classLoader = ClassLoaderFactory.createClassLoader

            (locations, types, parent);

tomcat自定义了一个类加载器，并且定义了ClassLoaderFactory根据参数定义返回classloader。总体上ClassLoaderFactory就是根据catalina.properties中的jar的位置，做了一些字符串的处理。

public static ClassLoader createClassLoader(String locations[],
                                                Integer types[],
                                                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>();

        if (locations != null && types != null && locations.length == types.length) {
            for (int i = 0; i < locations.length; i++)  {
                String location = locations[i];
                if ( types[i] == IS_URL ) {
                    URL url = new URL(location);
                    if (log.isDebugEnabled())
                        log.debug("  Including URL " + url);
                    set.add(url);
                } else if ( types[i] == IS_DIR ) {
                    File directory = new File(location);
                    directory = new File(directory.getCanonicalPath());
                    if (!directory.exists() || !directory.isDirectory() ||
                        !directory.canRead())
                         continue;
                    URL url = directory.toURI().toURL();
                    if (log.isDebugEnabled())
                        log.debug("  Including directory " + url);
                    set.add(url);
                } else if ( types[i] == IS_JAR ) {
                    File file=new File(location);
                    file = new File(file.getCanonicalPath());
                    if (!file.exists() || !file.canRead())
                        continue;
                    URL url = file.toURI().toURL();
                    if (log.isDebugEnabled())
                        log.debug("  Including jar file " + url);
                    set.add(url);
                } else if ( types[i] == IS_GLOB ) {
                    File directory=new File(location);
                    if (!directory.exists() || !directory.isDirectory() ||
                        !directory.canRead())
                        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 = new File(file.getCanonicalPath());
                        if (!file.exists() || !file.canRead())
                            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
        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]);
            }
        StandardClassLoader classLoader = null;
        if (parent == null)
            classLoader = new StandardClassLoader(array);
        else
            classLoader = new StandardClassLoader(array, parent);
        return (classLoader);

    }

 定义了类加载器后，在init()中加载

Class<?> startupClass =
            catalinaLoader.loadClass
            ("org.apache.catalina.startup.Catalina");

 这个才是tomcat的守护进程。

Thread.currentThread().setContextClassLoader(catalinaLoader);

        SecurityClassLoad.securityClassLoad(catalinaLoader);

        // Load our startup class and call its process() method
        if (log.isDebugEnabled())
            log.debug("Loading startup class");
        Class<?> startupClass =
            catalinaLoader.loadClass
            ("org.apache.catalina.startup.Catalina");
        Object startupInstance = startupClass.newInstance();        
        // Set the shared extensions class loader
        if (log.isDebugEnabled())
            log.debug("Setting startup class properties");
        String methodName = "setParentClassLoader";
        Class<?> paramTypes[] = new Class[1];
        paramTypes[0] = Class.forName("java.lang.ClassLoader");
        Object paramValues[] = new Object[1];
        paramValues[0] = sharedLoader;
        Method method =
            startupInstance.getClass().getMethod(methodName, paramTypes);
        method.invoke(startupInstance, paramValues);
        catalinaDaemon = startupInstance;

 这里要说的是

Class.forName()和ClassLoader.loadClass()的区别？

这两个方法由给类名作为参数，动态的定位并且加载类。然而，两者行为的区别在于用哪个加载器(java.lang.ClassLoader)去加载和加载完后的类是否就已经初始化。

对于Class.forName()最常见的形式就是用一个单独的String为参数，使用当前调用者的类加载器。这个类加载器加载代码执行forName()方法。比较ClassLoader.loadClass()，它是一个实例方法，需要你去选择确定的classloader。这个类加载器可以是也可以不是当前加载器。如果选择一个特定的类加载器对你的设计尤为重要，你可以使用ClassLoader.loadClass()或者三个参数的forName()。

更进一步，Class.forName()的常见形式会初始化加载的类。这样做的就是执行了类的静态初始化方法，也就是byte代码对应的所有静态初始化表达式。这和ClassLoader.loadClass()是不同的，ClassLoader.loadClass()直到类第一次使用的时候才初始化。