一、前言
类加载器,其实是很复杂一个东西,想等到我完全什么都弄明白了再写出来,估计不太现实。。。现在只能是知道多少写多少吧。
首先,我提一个问题:在我们自己的servlet中(比如ssm中,controller的代码),可以访问 tomcat 安装目录下 lib 中的类吗?(servlet-api.jar包中的不算)
好好思考一下再回答。如果你说不可以,那可能接下来会有点尴尬。。。
二、测试
1、tomcat 类加载器结构复习
咱们看图说话,应用程序类加载器,主要加载classpath路径下的类,在tomcat 的启动脚本里,最终会设置为 bin 目录下的bootstrap.jar 和tomcat-juli.jar:
common类加载器主要用于加载 tomcat 中间件自身、webapp 都可以访问的类;
catalina 类加载器,主要用于加载 tomcat 自身的类, webapp 不能访问;
共享类(shared)类加载器, 主要是用于加载 webapp 共享的类,比如大家都用 spring 开发,该类加载器的初衷就是加载 共用的 spring 相关的jar包。
这三者的加载路径,可以查看 Tomcat (我这边是Tomcat 8)安装目录下,conf / catalina.properties:
1 # 2 # 3 # List of comma-separated paths defining the contents of the "common" 4 # classloader. Prefixes should be used to define what is the repository type. 5 # Path may be relative to the CATALINA_HOME or CATALINA_BASE path or absolute. 6 # If left as blank,the JVM system loader will be used as Catalina's "common" 7 # loader. 8 # Examples: 9 # "foo": Add this folder as a class repository 10 # "foo/*.jar": Add all the JARs of the specified folder as class 11 # repositories 12 # "foo/bar.jar": Add bar.jar as a class repository 13 # 14 # Note: Values are enclosed in double quotes ("...") in case either the 15 # ${catalina.base} path or the ${catalina.home} path contains a comma. 16 # Because double quotes are used for quoting, the double quote character 17 # may not appear in a path. 18 common.loader="${catalina.base}/lib","${catalina.base}/lib/*.jar","${catalina.home}/lib","${catalina.home}/lib/*.jar" 19 20 # 21 # List of comma-separated paths defining the contents of the "server" 22 # classloader. Prefixes should be used to define what is the repository type. 23 # Path may be relative to the CATALINA_HOME or CATALINA_BASE path or absolute. 24 # If left as blank, the "common" loader will be used as Catalina's "server" 25 # loader. 26 # Examples: 27 # "foo": Add this folder as a class repository 28 # "foo/*.jar": Add all the JARs of the specified folder as class 29 # repositories 30 # "foo/bar.jar": Add bar.jar as a class repository 31 # 32 # Note: Values may be enclosed in double quotes ("...") in case either the 33 # ${catalina.base} path or the ${catalina.home} path contains a comma. 34 # Because double quotes are used for quoting, the double quote character 35 # may not appear in a path. 36 server.loader= 37 38 # 39 # List of comma-separated paths defining the contents of the "shared" 40 # classloader. Prefixes should be used to define what is the repository type. 41 # Path may be relative to the CATALINA_BASE path or absolute. If left as blank, 42 # the "common" loader will be used as Catalina's "shared" loader. 43 # Examples: 44 # "foo": Add this folder as a class repository 45 # "foo/*.jar": Add all the JARs of the specified folder as class 46 # repositories 47 # "foo/bar.jar": Add bar.jar as a class repository 48 # Please note that for single jars, e.g. bar.jar, you need the URL form 49 # starting with file:. 50 # 51 # Note: Values may be enclosed in double quotes ("...") in case either the 52 # ${catalina.base} path or the ${catalina.home} path contains a comma. 53 # Because double quotes are used for quoting, the double quote character 54 # may not appear in a path. 55 shared.loader=
但是,应该是从 tomcat 7开始, common.loader 和 shared.loader 已经默认置空了。 为什么留空的原因,这里先不详细讲述。(因为我也不完全懂啊,哈哈哈)
Webapp 类加载器就不用说了, 主要是加载自身目录下的 WEB-INF/classes、 WEB-INF/lib 中的类。
对这部分感兴趣的,可以再看看我的另一篇文章:实战分析Tomcat的类加载器结构(使用Eclipse MAT验证)
我们再回头看看,文章开头的图里,清晰地展示了: webapp的类加载器的parent,即为 common 类加载器。 那么,只要我们在 业务代码里进行如下调用,应该就获取到了 common 类加载器,于是就可以愉快地加载 Tomcat 安装目录下的 lib目录的jar了:
ClassLoader classLoader = this.getClass().getClassLoader(); ClassLoader directparent = classLoader.getParent();
2、验证程序
我这边建了个简单的web程序,只有一个servlet。
MyServlet .java:
1 import javax.servlet.*; 2 import java.io.IOException; 3 import java.lang.reflect.InvocationTargetException; 4 import java.lang.reflect.Method; 5 import java.net.URL; 6 import java.net.URLClassLoader; 7 15 public class MyServlet implements Servlet { 16 @Override 17 public void init(ServletConfig config) throws ServletException { 18 19 } 20 21 @Override 22 public ServletConfig getServletConfig() { 23 return null; 24 } 25 26 27 @Override 28 public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException { 29 ClassLoader classLoader = this.getClass().getClassLoader(); 30 System.out.println("当前类加载器(webapp加载器):" + classLoader); 31 printPath(classLoader); 32 33 ClassLoader directparent = classLoader.getParent(); 34 System.out.println("父加载器(tomcat 自身的加载器):" + directparent); 35 printPath(directparent); 36 37 // 从父加载器开始循环,应该会按顺序取到:应用类加载器--ext类加载器--bootstrap加载器 38 classLoader = directparent; 39 while (classLoader != null){ 40 ClassLoader parent = classLoader.getParent(); 41 System.out.println("当前类加载器为:" + parent); 42 printPath(parent); 43 classLoader = parent; 44 } 45 46 if (directparent != null) { 47 try { 48 Class<?> loadClass = directparent.loadClass("org.apache.catalina.core.StandardEngine"); 49 Object instance = loadClass.newInstance(); 50 Method[] methods = loadClass.getMethods(); 51 System.out.println("以下为StandardEngine的所有方法................."); 52 for (Method method : methods) { 53 System.out.println(method); 54 } 55 56 System.out.println("反射调用方法测试............................"); 57 Method getDefaultHostMethod = loadClass.getMethod("getDefaultHost"); 58 Object result = getDefaultHostMethod.invoke(instance); 59 System.out.println("before:" + result); 60 61 Method setDefaultHostMethod = loadClass.getMethod("setDefaultHost", String.class); 62 setDefaultHostMethod.invoke(instance,"hahaha..."); 63 64 Object afterResult = getDefaultHostMethod.invoke(instance); 65 System.out.println("after:" + afterResult); 66 67 } catch (ClassNotFoundException | IllegalAccessException | InstantiationException | InvocationTargetException | NoSuchMethodException e) { 68 e.printStackTrace(); 69 } 70 } 71 } 72 73 private void printPath(ClassLoader directparent) { 74 if (directparent instanceof URLClassLoader){ 75 URLClassLoader urlClassLoader = (URLClassLoader) directparent; 76 URL[] urLs = urlClassLoader.getURLs(); 77 for (URL urL : urLs) { 78 System.out.println(urL); 79 } 80 } 81 } 82 83 @Override 84 public String getServletInfo() { 85 return null; 86 } 87 88 @Override 89 public void destroy() { 90 91 } 92 }
加入到 web.xml中:
<servlet> <servlet-name>MyServlet</servlet-name> <servlet-class>MyServlet</servlet-class> <load-on-startup>1</load-on-startup> </servlet> <servlet-mapping> <servlet-name>MyServlet</servlet-name> <url-pattern>/*</url-pattern> </servlet-mapping>
pom.xml:
1 <?xml version="1.0" encoding="UTF-8"?> 2 3 <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 4 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> 5 <modelVersion>4.0.0</modelVersion> 6 7 <groupId>com.ckl</groupId> 8 <artifactId>tomcatclassloader</artifactId> 9 <version>1.0-SNAPSHOT</version> 10 <packaging>war</packaging> 11 12 <name>tomcatclassloader Maven Webapp</name> 13 <!-- FIXME change it to the project's website --> 14 <url>http://www.example.com</url> 15 16 <properties> 17 <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding> 18 <maven.compiler.source>1.7</maven.compiler.source> 19 <maven.compiler.target>1.7</maven.compiler.target> 20 </properties> 21 22 <dependencies> 23 24 <dependency> 25 <groupId>javax.servlet</groupId> 26 <artifactId>javax.servlet-api</artifactId> 27 <version>3.1.0</version> 28 <scope>provided</scope> 29 </dependency> 30 31 </dependencies> 32 33 <build> 34 <finalName>tomcatclassloader</finalName> 35 <pluginManagement><!-- lock down plugins versions to avoid using Maven defaults (may be moved to parent pom) --> 36 <plugins> 37 <plugin> 38 <artifactId>maven-clean-plugin</artifactId> 39 <version>3.1.0</version> 40 </plugin> 41 <!-- see http://maven.apache.org/ref/current/maven-core/default-bindings.html#Plugin_bindings_for_war_packaging --> 42 <plugin> 43 <artifactId>maven-resources-plugin</artifactId> 44 <version>3.0.2</version> 45 </plugin> 46 <plugin> 47 <artifactId>maven-compiler-plugin</artifactId> 48 <version>3.8.0</version> 49 </plugin> 50 <plugin> 51 <artifactId>maven-surefire-plugin</artifactId> 52 <version>2.22.1</version> 53 </plugin> 54 <plugin> 55 <artifactId>maven-war-plugin</artifactId> 56 <version>3.2.2</version> 57 </plugin> 58 <plugin> 59 <artifactId>maven-install-plugin</artifactId> 60 <version>2.5.2</version> 61 </plugin> 62 <plugin> 63 <artifactId>maven-deploy-plugin</artifactId> 64 <version>2.8.2</version> 65 </plugin> 66 </plugins> 67 </pluginManagement> 68 </build> 69 </project>
运行结果如下:
当前类加载器(webapp加载器):ParallelWebappClassLoader context: tomcatclassloader delegate: false ----------> Parent Classloader: java.net.URLClassLoader@1372ed45 file:/F:/ownprojects/tomcatclassloader/target/tomcatclassloader/WEB-INF/classes/ 父加载器(tomcat 自身的加载器):java.net.URLClassLoader@1372ed45 file:/D:/soft/apache-tomcat-8.5.23/lib/ file:/D:/soft/apache-tomcat-8.5.23/lib/annotations-api.jar file:/D:/soft/apache-tomcat-8.5.23/lib/catalina-ant.jar file:/D:/soft/apache-tomcat-8.5.23/lib/catalina-ha.jar file:/D:/soft/apache-tomcat-8.5.23/lib/catalina-storeconfig.jar file:/D:/soft/apache-tomcat-8.5.23/lib/catalina-tribes.jar file:/D:/soft/apache-tomcat-8.5.23/lib/catalina.jar file:/D:/soft/apache-tomcat-8.5.23/lib/ecj-4.6.3.jar file:/D:/soft/apache-tomcat-8.5.23/lib/el-api.jar file:/D:/soft/apache-tomcat-8.5.23/lib/jasper-el.jar file:/D:/soft/apache-tomcat-8.5.23/lib/jasper.jar file:/D:/soft/apache-tomcat-8.5.23/lib/jaspic-api.jar file:/D:/soft/apache-tomcat-8.5.23/lib/jsp-api.jar file:/D:/soft/apache-tomcat-8.5.23/lib/servlet-api.jar file:/D:/soft/apache-tomcat-8.5.23/lib/tomcat-api.jar file:/D:/soft/apache-tomcat-8.5.23/lib/tomcat-coyote.jar 。。。此处省略部分。。。 当前类加载器为:sun.misc.Launcher$AppClassLoader@18b4aac2 file:/D:/soft/apache-tomcat-8.5.23/bin/bootstrap.jar file:/D:/soft/apache-tomcat-8.5.23/bin/tomcat-juli.jar 当前类加载器为:sun.misc.Launcher$ExtClassLoader@43d7741f file:/C:/Program%20Files/Java/jdk1.8.0_172/jre/lib/ext/access-bridge-64.jar file:/C:/Program%20Files/Java/jdk1.8.0_172/jre/lib/ext/cldrdata.jar 。。。此处省略部分。。。 当前类加载器为:null 以下为StandardEngine的所有方法................. public void org.apache.catalina.core.StandardEngine.setParent(org.apache.catalina.Container) public org.apache.catalina.Service org.apache.catalina.core.StandardEngine.getService() 。。。此处省略部分。。。 反射调用方法测试............................ before:null after:hahaha...
由上可知,我们访问tomcat 自身的类,比如 org.apache.catalina.core.StandardEngine,是完全没问题的。
二、可怕的参数传递实验
1、实验思路
不太好描述,直接码,我们首先定义一个测试类,
# TestSample.java public class TestSample { public void printClassLoader(TestSample testSample) { System.out.println(testSample.getClass().getClassLoader()); } }
这个类,足够简单,里面仅一个方法,方法接收一个自己类型的参数,方法体是打印出参数的类加载器。
在测试类中,直接 new 一个该类的对象A,然后调用其 printClassLoader,将对象A自己传入,默认的打印结果是:
TestSample loader = new TestSample(); loader.printClassLoader(loader);
sun.misc.Launcher$AppClassLoader@18b4aac2
sun.misc.Launcher$AppClassLoader 这个类,就是我们的应用类加载器,一般程序里,没有显示定义过类加载器的话,classpath下的类都由该类加载。
我们要做的试验有两个:
1、如果传入的参数对象,由另外一个类加载器加载的,能调用成功吗,如果成功,结果是什么?
2、如果由两个相同类加载器的不同实例,来加载 TestSample ,然后反射获取对象,那么其中一个能作为另一个对象的 printClassLoader 的参数吗?
开始之前,先准备好我们自定义的类加载器,
1 import java.io.ByteArrayOutputStream; 2 import java.io.FileInputStream; 3 import java.io.UnsupportedEncodingException; 4 5 /** 6 * desc: 7 * 8 * @author : caokunliang 9 * creat_date: 2019/6/13 0013 10 * creat_time: 10:19 11 **/ 12 public class MyClassLoader extends ClassLoader { 13 private String classPath; 14 private String className; 15 16 17 public MyClassLoader(String classPath, String className) { 18 this.classPath = classPath; 19 this.className = className; 20 } 21 22 @Override 23 protected Class<?> findClass(String name) throws ClassNotFoundException { 24 byte[] data = getData(); 25 try { 26 String string = new String(data, "utf-8"); 27 System.out.println(string); 28 } catch (UnsupportedEncodingException e) { 29 e.printStackTrace(); 30 } 31 32 return defineClass(className,data,0,data.length); 33 } 34 35 private byte[] getData(){ 36 String path = classPath; 37 38 try { 39 FileInputStream inputStream = new FileInputStream(path); 40 ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream(); 41 byte[] bytes = new byte[2048]; 42 int num = 0; 43 while ((num = inputStream.read(bytes)) != -1){ 44 byteArrayOutputStream.write(bytes, 0,num); 45 } 46 47 return byteArrayOutputStream.toByteArray(); 48 } catch (Exception e) { 49 e.printStackTrace(); 50 } 51 52 return null; 53 } 54 }
使用方法就像下面这样:
MyClassLoader classLoader = new MyClassLoader("F:\\ownprojects\\test\\out\\TestSample.class", className); Class<?> loadClass = classLoader.findClass(className);
2、实验1:应用默认加载器 && 自定义加载器
import java.lang.reflect.InvocationTargetException; import java.lang.reflect.Method; /** * desc: * * @author : caokunliang * creat_date: 2019/6/14 0014 * creat_time: 17:04 **/ public class MainTest {
public static void testMyClassLoaderAndAppClassloader()throws Exception{ // TestSample类由sun.misc.Launcher$AppClassLoader 加载,那么 printClassLoader 需要的参数类型应该也是 Launcher$AppClassLoader加载的TestSample类型 // 而这里的 sample 正好满足,所以可以成功 TestSample sample = new TestSample(); sample.printClassLoader(sample); String className = "TestSample"; MyClassLoader classLoader = new MyClassLoader("F:\\ownprojects\\test\\out\\TestSample.class", className); Class<?> loadClass = classLoader.findClass(className); Object instance = loadClass.newInstance(); // 查看是否能赋值 System.out.println(sample.getClass().isAssignableFrom(loadClass) ); //1 error: 这里会报错哦 TestSample instance1 = (TestSample) instance; sample.printClassLoader(instance1); } public static void main(String[] args) throws Exception { testMyClassLoaderAndAppClassloader(); } }
执行结果如下,在上图1处,会报错,错误为转型错误:
[Loaded TestSample from __JVM_DefineClass__]
Exception in thread "main" java.lang.ClassCastException: TestSample cannot be cast to TestSample
at MainTest.testMyClassLoaderAndAppClassloader(MainTest.java:25)
at MainTest.main(MainTest.java:48)
这里可以看出来,不同类加载器加载的类,即使是同一个类,也是不兼容的。因为这个例子中,一个是由Launcher$AppClassLoader加载,一个是自定义加载器加载的。
下面,我们将进一步验证这个结论。
3、实验2:自定义加载器 && 自定义加载器 (不同实例)
实验 3-1:
public static void testMyClassLoaderAndAnotherMyClassLoader() throws Exception { String className = "TestSample"; MyClassLoader classLoader = new MyClassLoader("F:\\ownprojects\\test\\out\\TestSample.class", className); Class<?> loadClass = classLoader.findClass(className); Object instance = loadClass.newInstance(); MyClassLoader classLoader1 = new MyClassLoader("F:\\ownprojects\\test\\out\\TestSample.class", className); Class<?> loadClass1 = classLoader1.findClass(className); Object instance1 = loadClass1.newInstance(); // 1 Method method = instance.getClass().getMethod("printClassLoader", new Class[]{TestSample.class}); method.invoke(instance,instance); }
上图1处,会报错,报错如下,原因是TestSample.class 默认在classpath下,由应用类加载器加载,而 instance 是由 classLoader 加载的,参数类型因此不匹配:
Exception in thread "main" java.lang.NoSuchMethodException: TestSample.printClassLoader(TestSample) at java.lang.Class.getMethod(Class.java:1786) at MainTest.testMyClassLoaderAndAnotherMyClassLoader(MainTest.java:43) at MainTest.main(MainTest.java:49)
实验 3-2:
(改动仅标红处)
public static void testMyClassLoaderAndAnotherMyClassLoader() throws Exception { String className = "TestSample"; MyClassLoader classLoader = new MyClassLoader("F:\\ownprojects\\test\\out\\TestSample.class", className); Class<?> loadClass = classLoader.findClass(className); Object instance = loadClass.newInstance(); MyClassLoader classLoader1 = new MyClassLoader("F:\\ownprojects\\test\\out\\TestSample.class", className); Class<?> loadClass1 = classLoader1.findClass(className); Object instance1 = loadClass1.newInstance(); Method method = instance.getClass().getMethod("printClassLoader", new Class[]{loadClass}); method.invoke(instance,instance); }
可以正常执行,结果为:
[Loaded TestSample from __JVM_DefineClass__]
MyClassLoader@41a4555e
实验3-3:
public static void testMyClassLoaderAndAnotherMyClassLoader() throws Exception { String className = "TestSample"; MyClassLoader classLoader = new MyClassLoader("F:\\ownprojects\\test\\out\\TestSample.class", className); Class<?> loadClass = classLoader.findClass(className); Object instance = loadClass.newInstance(); MyClassLoader classLoader1 = new MyClassLoader("F:\\ownprojects\\test\\out\\TestSample.class", className); Class<?> loadClass1 = classLoader1.findClass(className); Object instance1 = loadClass1.newInstance(); Method method = instance.getClass().getMethod("printClassLoader", new Class[]{loadClass1}); method.invoke(instance,instance); }
报错,错误和实验3-1差不多:
Exception in thread "main" java.lang.NoSuchMethodException: TestSample.printClassLoader(TestSample) at java.lang.Class.getMethod(Class.java:1786) at MainTest.testMyClassLoaderAndAnotherMyClassLoader(MainTest.java:43) at MainTest.main(MainTest.java:49)
实验3-4:
public static void testMyClassLoaderAndAnotherMyClassLoader() throws Exception { String className = "TestSample"; MyClassLoader classLoader = new MyClassLoader("F:\\ownprojects\\test\\out\\TestSample.class", className); Class<?> loadClass = classLoader.findClass(className); Object instance = loadClass.newInstance(); MyClassLoader classLoader1 = new MyClassLoader("F:\\ownprojects\\test\\out\\TestSample.class", className); Class<?> loadClass1 = classLoader1.findClass(className); Object instance1 = loadClass1.newInstance(); Method method = instance.getClass().getMethod("printClassLoader", new Class[]{loadClass}); method.invoke(instance,instance1); }
此时报错和前面不同:
Exception in thread "main" java.lang.IllegalArgumentException: argument type mismatch at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at MainTest.testMyClassLoaderAndAnotherMyClassLoader(MainTest.java:44) at MainTest.main(MainTest.java:49)
好了,做了这么多实验,想必大概都了解了吧,参数类型不只是完全限定类名要一致,而且还需要类加载器一致才行。
简单的参数传递,实际上隐藏了如此之多的东西。参数要传对,看来不能拼人品啊,还是得靠知识。
三、关于Tomcat 中类加载器的思考
不知道看完了上面的实验,大家有没有想到一个问题,在我们的servlet 开发中,servlet-api.jar 包默认是由 tomcat 提供的,意思也就是,servlet-api.jar中的类应该都是由 tomcat 的common 类加载器加载的。(这个早已验证,可翻我之前的博客)
servlet-api.jar包中,有很多类,大家肯定用过 javax.servlet.Filter#doFilter :
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException;
我们思考一个问题,假如 我们在我们 web-inf/lib下,自己放上一个 servlet-api.jar,那么加载 web-inf/lib 的自然就是 webappClassloader,那么加载我们的filter的,也就是 webappClassloader。那么我们的filter的参数,默认就应该只接受 webappclassloader 加载的 ServletRequest 、ServletResponse 类。
但是,很显然,因为 Tomcat 的lib下面也有 servlet-api.jar,给我们的filter 传递的 reqeust参数,应该是由其 自己的common 类加载器加载的,问题来了,这样还能调用成功我们的 filter 方法吗?按理说,不可能,应该会报一个参数类型不匹配的错误才对,因为上一章的实验结果就摆在那里。
那就再测试一次吧,事实胜于雄辩,首先,我们将复用第一章的例子的servlet,唯一要改的,只是pom.xml(注释了provided那行):
1 <dependency> 2 <groupId>javax.servlet</groupId> 3 <artifactId>javax.servlet-api</artifactId> 4 <version>3.1.0</version> 5 <!--<scope>provided</scope>--> 6 </dependency>
maven打包部署到tomcat,我们启动Tomcat时,可以在catalina.sh/bat 中加一个参数:-XX:+TraceClassLoading,启动后,访问我们的 MyServlet,并没有什么异常(大家可以试试)。
然后我看了下,servletRequest等class,到底从哪加载的,下图可以看出来,都是来自 tomcat 自身的 servlet-api.jar包:
而我们的 web-inf下的 servlet-api 包,完全就是个悲剧,被忽略了啊。。。惨。。。(我要你有何用??)
而且,另外一个层面来说,运行完全没报错,说明 webapp 中 加载servlet-api.jar包的classloader 和 tomcat 加载 servlet-api.jar包的classloader 为同一个,不然早就报错了。那么意思就是说, webapp 中加载 servlet-api.jar ,其实用的 tomcat 的common 类加载器去加载。(我真的柯南附体了。。。) 反证法也可以说明这一点,因为我们在 webapp的lib 下,是可以不放 servlet-api.jar包的,jar包只在 tomcat 有,而 webapp 的类加载器又不能去加载 tomcat 的东西,所以,只能说: webapp 类加载器委托了 tomcat 帮他加载。
我们可以看看 webappclassloader 的实现,我本地源码版本是 tomcat 7的,不过无所谓,都差不多:
org.apache.catalina.loader.WebappClassLoaderBase#loadClass(java.lang.String, boolean):
1 synchronized (getClassLoadingLockInternal(name)) { 2 if (log.isDebugEnabled()) 3 log.debug("loadClass(" + name + ", " + resolve + ")"); 4 Class<?> clazz = null; 5 6 7 // (0) Check our previously loaded local class cache // 检查本加载器是否加载过了,本地有个map 8 clazz = findLoadedClass0(name); 9 if (clazz != null) { 10 if (log.isDebugEnabled()) 11 log.debug(" Returning class from cache"); 12 if (resolve) 13 resolveClass(clazz); 14 return (clazz); 15 } 16 17 // (0.1) Check our previously loaded class cache // 调用了本加载器的本地方法,查看是否加载过了 18 clazz = findLoadedClass(name); 19 if (clazz != null) { 20 if (log.isDebugEnabled()) 21 log.debug(" Returning class from cache"); 22 if (resolve) 23 resolveClass(clazz); 24 return (clazz); 25 } 26 27 // (0.2) Try loading the class with the system class loader, to prevent // 先交给 扩展类加载器,免得把 jre/ext下面的类自己加载了出大事 28 // the webapp from overriding J2SE classes 29 try { 30 clazz = j2seClassLoader.loadClass(name); 31 if (clazz != null) { 32 if (resolve) 33 resolveClass(clazz); 34 return (clazz); 35 } 36 } catch (ClassNotFoundException e) { 37 // Ignore 38 } 39 40 // (0.5) Permission to access this class when using a SecurityManager 这个不管,我们这边是null 41 if (securityManager != null) { 42 int i = name.lastIndexOf('.'); 43 if (i >= 0) { 44 try { 45 securityManager.checkPackageAccess(name.substring(0,i)); 46 } catch (SecurityException se) { 47 String error = "Security Violation, attempt to use " + 48 "Restricted Class: " + name; 49 log.info(error, se); 50 throw new ClassNotFoundException(error, se); 51 } 52 } 53 } 54 // 000 55 boolean delegateLoad = delegate || filter(name); //默认为false,可以配置,如果为true,表示应该交给 tomcat 的common类加载器先加载 56 57 // (1) Delegate to our parent if requested 58 if (delegateLoad) { 59 if (log.isDebugEnabled()) 60 log.debug(" Delegating to parent classloader1 " + parent); 61 try { 62 clazz = Class.forName(name, false, parent); 63 if (clazz != null) { 64 if (log.isDebugEnabled()) 65 log.debug(" Loading class from parent"); 66 if (resolve) 67 resolveClass(clazz); 68 return (clazz); 69 } 70 } catch (ClassNotFoundException e) { 71 // Ignore 72 } 73 } 74 75 // (2) Search local repositories // 如果 tomcat 的common类加载器 加载失败,则有自己加载 76 if (log.isDebugEnabled()) 77 log.debug(" Searching local repositories"); 78 try { 79 clazz = findClass(name); 80 if (clazz != null) { 81 if (log.isDebugEnabled()) 82 log.debug(" Loading class from local repository"); 83 if (resolve) 84 resolveClass(clazz); 85 return (clazz); 86 } 87 } catch (ClassNotFoundException e) { 88 // Ignore 89 } 90 91 // (3) Delegate to parent unconditionally // 如果自己加载失败了,别说了,都甩给 tomcat 的common类加载器吧 92 if (!delegateLoad) { 93 if (log.isDebugEnabled()) 94 log.debug(" Delegating to parent classloader at end: " + parent); 95 try { 96 clazz = Class.forName(name, false, parent); 97 if (clazz != null) { 98 if (log.isDebugEnabled()) 99 log.debug(" Loading class from parent"); 100 if (resolve) 101 resolveClass(clazz); 102 return (clazz); 103 } 104 } catch (ClassNotFoundException e) { 105 // Ignore 106 } 107 }
说下上面的000处,这里filter(name),会判断要加载的类,是否是javax.servlet这样的包名,比如,servlet-api.jar包的类,就满足这里的条件:
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
1 protected boolean filter(String name, boolean isClassName) { 2 3 if (name == null) 4 return false; 5 6 char ch; 7 if (name.startsWith("javax")) { 8 /* 5 == length("javax") */ 9 if (name.length() == 5) { 10 return false; 11 } 12 ch = name.charAt(5); 13 if (isClassName && ch == '.') { 14 /* 6 == length("javax.") */ 15 if (name.startsWith("servlet.jsp.jstl.", 6)) { 16 return false; 17 } 18 if (name.startsWith("el.", 6) || 19 name.startsWith("servlet.", 6) || 20 name.startsWith("websocket.", 6) || 21 name.startsWith("security.auth.message.", 6)) { 22 return true; 23 } 24 } else if (!isClassName && ch == '/') { 25 /* 6 == length("javax/") */ 26 if (name.startsWith("servlet/jsp/jstl/", 6)) { 27 return false; 28 } 29 if (name.startsWith("el/", 6) || 30 name.startsWith("servlet/", 6) || 31 name.startsWith("websocket/", 6) || 32 name.startsWith("security/auth/message/", 6)) { 33 return true; 34 } 35 } 36 } else if (name.startsWith("org")) { 37 /* 3 == length("org") */ 38 if (name.length() == 3) { 39 return false; 40 } 41 ch = name.charAt(3); 42 if (isClassName && ch == '.') { 43 /* 4 == length("org.") */ 44 if (name.startsWith("apache.", 4)) { 45 /* 11 == length("org.apache.") */ 46 if (name.startsWith("tomcat.jdbc.", 11)) { 47 return false; 48 } 49 if (name.startsWith("el.", 11) || 50 name.startsWith("catalina.", 11) || 51 name.startsWith("jasper.", 11) || 52 name.startsWith("juli.", 11) || 53 name.startsWith("tomcat.", 11) || 54 name.startsWith("naming.", 11) || 55 name.startsWith("coyote.", 11)) { 56 return true; 57 } 58 } 59 } else if (!isClassName && ch == '/') { 60 /* 4 == length("org/") */ 61 if (name.startsWith("apache/", 4)) { 62 /* 11 == length("org/apache/") */ 63 if (name.startsWith("tomcat/jdbc/", 11)) { 64 return false; 65 } 66 if (name.startsWith("el/", 11) || 67 name.startsWith("catalina/", 11) || 68 name.startsWith("jasper/", 11) || 69 name.startsWith("juli/", 11) || 70 name.startsWith("tomcat/", 11) || 71 name.startsWith("naming/", 11) || 72 name.startsWith("coyote/", 11)) { 73 return true; 74 } 75 } 76 } 77 } 78 return false; 79 }
简单归纳下:
1、webappclassloader 加载时,先看本加载器的缓存,看看是否加载过了,加载过了直接返回,否则进入2;
2、先给 jdk 的jre/ext 类加载器加载, jre/ext 如果加载不了,会丢给 Bootstrap 加载器,如果加载到了,则返回,否则进入3;
3、判断delegate 属性,如果为true,则进入3.1,为false,则进入 3.2
3.1 如果要加载的类,包名大概是javax.servlet开头,delegate会为true,就会丢给tomcat 的common 类加载器,加载成功则返回,否则本加载器真正尝试加载,成功则返回,否则抛异常:加载失败。
3.2 先让自己类加载器尝试,成功则返回,否则丢给 tomcat 加载,成功则返回,否则抛异常:加载失败。
四、总结
对象,由类生成,类,由类加载器加载而来。 对象的方法参数的类型,也和类加载器息息相关, 这个参数是 类加载器 A 加载的class B类型,你必须也传一个这样的给我,我才认啊。
举个例子,假设你先后有过两个女朋友,前女友给你送了个iphone 8,现女友也送了你一个iphone 8, 这两个iphone 8 都是同一个地方买的,那这两个iPhone 8 能一样吗?要不问问你现女友去?
所以说啊,java这东西,他么的易学难精。。。继续努力吧。 下篇可以写写热部署、OSGI的问题,(半桶水,我自己也要去研究下,哈哈)。。