我目前的系统可能需要自己实现类加载器，想要参考Tomcat的实现。关于Tomcat的类加载机制，网上文章很多，当然大多都是互相copy，有价值的信息并不多，不得已我开始看Tomcat代码，略有所得，记录起来。主要针对WebappClassLoader。
负责Web应用的类加载的是org.apache.catalina.loader.WebappClassLoader，它几个比较重要的方 法：findClass(),loadClass(),findClassInternal(),findResourceInternal().类加载 器被用来加载一个类的时候，loadClass()会被调用，loadClass()则调用findClass()。后两个方法是 WebappClassLoader的私有方法，findClass()调用findClassInternal()来创建class对象，而 findClassInternal()则需要findResourceInternal()来查找.class文件。
通常自己实现类记载器只要实现findclass即可，这里为了实现特殊目的而override了loadClass().
下面是精简过的代码（去除了几乎全部关于log、异常和安全控制的代码）：
findClass:
1 public Class findClass(String name) throws ClassNotFoundException {
2 // 先试图自己加载类，找不到则请求parent来加载
3 // 注意这点和java默认的双亲委托模式不同
4 Class clazz = null;
5 clazz = findClassInternal(name);
6 if ((clazz == null) && hasExternalRepositories) {
7 synchronized (this) {
8 clazz = super.findClass(name);
9 }
10 }
11 if (clazz == null) {
12 throw new ClassNotFoundException(name);
13 }
14
15 return (clazz);
16 }
loadClass:
1 public Class loadClass(String name, boolean resolve)
2 throws ClassNotFoundException {
3 Class clazz = null;
4 // (0) 先从自己的缓存中查找，有则返回，无则继续
5 clazz = findLoadedClass0(name);
6 if (clazz != null) {
7 if (resolve) resolveClass(clazz);
8 return (clazz);
9 }
10 // (0.1) 再从parent的缓存中查找
11 clazz = findLoadedClass(name);
12 if (clazz != null) {
13 if (resolve) resolveClass(clazz);
14 return (clazz);
15 }
16 // (0.2) 缓存中没有，则首先使用system类加载器来加载
17 clazz = system.loadClass(name);
18 if (clazz != null) {
19 if (resolve) resolveClass(clazz);
20 return (clazz);
21 }
22 //判断是否需要先让parent代理
23 boolean delegateLoad = delegate || filter(name);
24 // (1) 先让parent加载，通常delegateLoad == false，即这一步不会执行
25
26 if (delegateLoad) {
27 ClassLoader loader = parent;
28 if (loader == null)
29 loader = system;
30 clazz = loader.loadClass(name);
31 if (clazz != null) {
32 if (resolve) resolveClass(clazz);
33 return (clazz);
34 }
35 }
36 // (2) delegateLoad == false 或者 parent加载失败，调用自身的加载机制
37 clazz = findClass(name);
38 if (clazz != null) {
39 if (resolve) resolveClass(clazz);
40 return (clazz);
41 }
42 // (3) 自己加载失败，则请求parent代理加载
43
44 if (!delegateLoad) {
45 ClassLoader loader = parent;
46 if (loader == null)
47 loader = system;
48 clazz = loader.loadClass(name);
49 if (clazz != null) {
50 return (clazz);
51 }
52 }
53 throw new ClassNotFoundException(name);
54 }
findClassInternal:
1 protected Class findClassInternal(String name)
2 throws ClassNotFoundException {
3 if (!validate(name))
4 throw new ClassNotFoundException(name);
5 //根据类名查找资源
6 String tempPath = name.replace('.', '/');
7 String classPath = tempPath + ".class";
8 ResourceEntry entry = null;
9 entry = findResourceInternal(name, classPath);
10
11 if (entry == null)
12 throw new ClassNotFoundException(name);
13 //如果以前已经加载成功过这个类，直接返回
14
15 Class clazz = entry.loadedClass;
16 if (clazz != null)
17 return clazz;
18 //以下根据找到的资源(.class文件)进行:1、定义package;2、对package安全检查;3、定义class，即创建class对象
19 synchronized (this) {
20 if (entry.binaryContent == null && entry.loadedClass == null)
21 throw new ClassNotFoundException(name);
22 // Looking up the package
23 String packageName = null;
24 int pos = name.lastIndexOf('.');
25 if (pos != -1)
26 packageName = name.substring(0, pos);
27 Package pkg = null;
28 if (packageName != null) {
29 pkg = getPackage(packageName);
30 // Define the package (if null)
31 if (pkg == null) {
32 //定义package的操作，此处省略，具体参看源码
33 pkg = getPackage(packageName);
34 }
35 }
36 if (securityManager != null) {
37 //安全检查操作，此处省略，具体参看源码
38 }
39 //创建class对象并返回
40 if (entry.loadedClass == null) {
41 try {
42 clazz = defineClass(name, entry.binaryContent, 0,
43 entry.binaryContent.length,
44 new CodeSource(entry.codeBase, entry.certificates));
45 } catch (UnsupportedClassVersionError ucve) {
46 throw new UnsupportedClassVersionError(
47 ucve.getLocalizedMessage() + " " +
48 sm.getString("webappClassLoader.wrongVersion",
49 name));
50 }
51 entry.loadedClass = clazz;
52 entry.binaryContent = null;
53 entry.source = null;
54 entry.codeBase = null;
55 entry.manifest = null;
56 entry.certificates = null;
57 } else {
58 clazz = entry.loadedClass;
59 }
60 }
61 return clazz;
62 }
findResouceInternal():
下几篇介绍WebappLoader,StandardContext,StandardWrapper,ApplicationDispatcher在类加载中的作用。其中ApplicationDispatcher是核心。
1 //要先加载相关实体资源(.jar) 再加载查找的资源本身
2 protected ResourceEntry findResourceInternal(String name, String path) {
3 //先根据类名从缓存中查找对应资源 ，有则直接返回
4 ResourceEntry entry = (ResourceEntry) resourceEntries.get(name);
5 if (entry != null)
6 return entry;
7 int contentLength = -1;//资源二进制数据长度
8 InputStream binaryStream = null;//资源二进制输入流
9
10 int jarFilesLength = jarFiles.length;//classpath中的jar包个数
11 int repositoriesLength = repositories.length;//仓库数(classpath每一段称为repository仓库)
12 int i;
13 Resource resource = null;//加载的资源实体
14 boolean fileNeedConvert = false;
15 //对每个仓库迭代，直接找到相应的entry，如果查找的资源是一个独立的文件，在这个代码块可以查找到相应资源，
16 //如果是包含在jar包中的类，这段代码并不能找出其对应的资源
17 for (i = 0; (entry == null) && (i < repositoriesLength); i++) {
18 try {
19 String fullPath = repositories[i] + path;//仓库路径 加资源路径得到全路径
20 Object lookupResult = resources.lookup(fullPath);//从资源库中查找资源
21
22 if (lookupResult instanceof Resource) {
23 resource = (Resource) lookupResult;
24 }
25 //到这里没有抛出异常，说明资源已经找到，现在构造entry对象
26 if (securityManager != null) {
27 PrivilegedAction dp =
28 new PrivilegedFindResource(files[i], path);
29 entry = (ResourceEntry)AccessController.doPrivileged(dp);
30 } else {
31 entry = findResourceInternal(files[i], path);//这个方式只是构造entry并给其codebase和source赋值
32 }
33 //获取资源长度和最后修改时间
34 ResourceAttributes attributes =
35 (ResourceAttributes) resources.getAttributes(fullPath);
36 contentLength = (int) attributes.getContentLength();
37 entry.lastModified = attributes.getLastModified();
38 //资源找到，将二进制输入流赋给binaryStream
39 if (resource != null) {
40 try {
41 binaryStream = resource.streamContent();
42 } catch (IOException e) {
43 return null;
44 }
45 //将资源的最后修改时间加到列表中去，代码略去，参加源码
46
47 }
48
49 } catch (NamingException e) {
50 }
51 }
52 if ((entry == null) && (notFoundResources.containsKey(name)))
53 return null;
54 //开始从jar包中查找
55 JarEntry jarEntry = null;
56 synchronized (jarFiles) {
57 if (!openJARs()) {
58 return null;
59 }
60 for (i = 0; (entry == null) && (i < jarFilesLength); i++) {
61 jarEntry = jarFiles[i].getJarEntry(path);//根据路径从jar包中查找资源
62 //如果jar包中找到资源，则定义entry并将二进制流等数据赋给entry，同时将jar包解压到workdir.
63 if (jarEntry != null) {
64 entry = new ResourceEntry();
65 try {
66 entry.codeBase = getURL(jarRealFiles[i], false);
67 String jarFakeUrl = getURI(jarRealFiles[i]).toString();
68 jarFakeUrl = "jar:" + jarFakeUrl + "!/" + path;
69 entry.source = new URL(jarFakeUrl);
70 entry.lastModified = jarRealFiles[i].lastModified();
71 } catch (MalformedURLException e) {
72 return null;
73 }
74 contentLength = (int) jarEntry.getSize();
75 try {
76 entry.manifest = jarFiles[i].getManifest();
77 binaryStream = jarFiles[i].getInputStream(jarEntry);
78 } catch (IOException e) {
79 return null;
80 }
81 if (antiJARLocking && !(path.endsWith(".class"))) {
82 //解压jar包代码，参见源码
83 }
84 }
85 }
86 if (entry == null) {
87 synchronized (notFoundResources) {
88 notFoundResources.put(name, name);
89 }
90 return null;
91 }
92 //从二进制流将资源内容读出
93 if (binaryStream != null) {
94 byte[] binaryContent = new byte[contentLength];
95 //读二进制流的代码，这里省去，参见源码
96 entry.binaryContent = binaryContent;
97 }
98 }
99 // 将资源加到缓存中
100 synchronized (resourceEntries) {
101 ResourceEntry entry2 = (ResourceEntry) resourceEntries.get(name);
102 if (entry2 == null) {
103 resourceEntries.put(name, entry);
104 } else {
105 entry = entry2;
106 }
107 }
108 return entry;
109 }