还是Tomcat,关于类加载器的趣味实验,实战分析Tomcat的类加载器结构(使用Eclipse MAT验证)

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2019-6-26

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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) ); // 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,在上图标红行,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();

        Method method = instance.getClass().getMethod("printClassLoader", new Class[]{TestSample.class});
        method.invoke(instance,instance);

    }

上图红色处,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     
 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             }

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  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)。。

 

作者:三国梦回