1. 程式人生 > >還是Tomcat,關於類載入器的趣味實驗

還是Tomcat,關於類載入器的趣味實驗

一、前言

類載入器,其實是很複雜一個東西,想等到我完全什麼都弄明白了再寫出來,估計不太現實。。。現在只能是知道多少寫多少吧。

首先,我提一個問題:在我們自己的servlet中(比如ssm中,controller的程式碼),可以訪問 tomcat 安裝目錄下 lib 中的類嗎?(servlet-api.jar包中的不算)

好好思考一下再回答。如果你說不可以,那可能接下來會有點尷尬。。。

 

二、測試

1、tomcat 類載入器結構複習

咱們看圖說話,應用程式類載入器,主要載入classpath路徑下的類,在tomcat 的啟動腳本里,最終會設定為 bin 目錄下的bootstrap.jar 和tomcat-juli.jar:

 

 common類載入器主要用於載入 tomcat 中介軟體自身、webapp 都可以訪問的類;

 catalina 類載入器,主要用於載入 tomcat 自身的類, webapp 不能訪問;

 共享類(shared)類載入器, 主要是用於載入 webapp 共享的類,比如大家都用 spring 開發,該類載入器的初衷就是載入 共用的 spring 相關的jar包。 

這三者的載入路徑,可以檢視 Tomcat (我這邊是Tomcat 8)安裝目錄下,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,應該是從 tomcat 7開始, common.loader 和 shared.loader 已經預設置空了。 為什麼留空的原因,這裡先不詳細講述。(因為我也不完全懂啊,哈哈哈)

 

Webapp 類載入器就不用說了, 主要是載入自身目錄下的 WEB-INF/classes、 WEB-INF/lib 中的類。

對這部分感興趣的,可以再看看我的另一篇文章:實戰分析Tomcat的類載入器結構(使用Eclipse MAT驗證)

 

我們再回頭看看,文章開頭的圖裡,清晰地展示了: webapp的類載入器的parent,即為 common 類載入器。 那麼,只要我們在 業務程式碼裡進行如下呼叫,應該就獲取到了 common 類載入器,於是就可以愉快地載入 Tomcat 安裝目錄下的 lib目錄的jar了:

        ClassLoader classLoader = this.getClass().getClassLoader();
        ClassLoader directparent = classLoader.getParent();

 

2、驗證程式

我這邊建了個簡單的web程式,只有一個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("當前類載入器(webapp載入器):" + classLoader);
31         printPath(classLoader);
32 
33         ClassLoader directparent = classLoader.getParent();
34         System.out.println("父載入器(tomcat 自身的載入器):" + directparent);
35         printPath(directparent);
36 
37         // 從父載入器開始迴圈,應該會按順序取到:應用類載入器--ext類載入器--bootstrap載入器
38         classLoader = directparent;
39         while (classLoader != null){
40             ClassLoader parent = classLoader.getParent();
41             System.out.println("當前類載入器為:" + 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("以下為StandardEngine的所有方法.................");
52                 for (Method method : methods) {
53                     System.out.println(method);
54                 }
55 
56                 System.out.println("反射呼叫方法測試............................");
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>

 

執行結果如下:

當前類載入器(webapp載入器):ParallelWebappClassLoader
  context: tomcatclassloader
  delegate: false
----------> Parent Classloader:
java.net.URLClassLoader@1372ed45

file:/F:/ownprojects/tomcatclassloader/target/tomcatclassloader/WEB-INF/classes/

父載入器(tomcat 自身的載入器):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
。。。此處省略部分。。。


當前類載入器為:sun.misc.Launcher$AppClassLoader@18b4aac2
file:/D:/soft/apache-tomcat-8.5.23/bin/bootstrap.jar
file:/D:/soft/apache-tomcat-8.5.23/bin/tomcat-juli.jar
當前類載入器為:sun.misc.Launcher$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
。。。此處省略部分。。。

當前類載入器為:null
以下為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()
。。。此處省略部分。。。

反射呼叫方法測試............................
before:null
after:hahaha...

 

由上可知,我們訪問tomcat 自身的類,比如 org.apache.catalina.core.StandardEngine,是完全沒問題的。

 

二、可怕的引數傳遞實驗

1、實驗思路

不太好描述,直接碼,我們首先定義一個測試類,

# TestSample.java
public class TestSample {

    public void printClassLoader(TestSample testSample) {
        System.out.println(testSample.getClass().getClassLoader());
    }
}

這個類,足夠簡單,裡面僅一個方法,方法接收一個自己型別的引數,方法體是打印出引數的類載入器。

 

在測試類中,直接 new 一個該類的物件A,然後呼叫其 printClassLoader,將物件A自己傳入,預設的列印結果是:

        TestSample loader = new TestSample();
        loader.printClassLoader(loader);
        sun.misc.Launcher$AppClassLoader@18b4aac2

 

sun.misc.Launcher$AppClassLoader 這個類,就是我們的應用類載入器,一般程式裡,沒有顯示定義過類載入器的話,classpath下的類都由該類載入。

我們要做的試驗有兩個:

1、如果傳入的引數物件,由另外一個類載入器載入的,能呼叫成功嗎,如果成功,結果是什麼?

2、如果由兩個相同類載入器的不同例項,來載入 TestSample ,然後反射獲取物件,那麼其中一個能作為另一個物件的 printClassLoader 的引數嗎?

 

開始之前,先準備好我們自定義的類載入器,

 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 }

 

使用方法就像下面這樣: 

        MyClassLoader classLoader = new MyClassLoader("F:\\\\ownprojects\\\\test\\\\out\\\\TestSample.class", className);
        Class<?> loadClass = classLoader.findClass(className);

 

2、實驗1:應用預設載入器 && 自定義載入器

 

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類由sun.misc.Launcher$AppClassLoader 載入,那麼 printClassLoader 需要的引數型別應該也是 Launcher$AppClassLoader載入的TestSample型別 // 而這裡的 sample 正好滿足,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(); // 檢視是否能賦值 System.out.println(sample.getClass().isAssignableFrom(loadClass) ); // error: 這裡會報錯哦 TestSample instance1 = (TestSample) instance; sample.printClassLoader(instance1); } public static void main(String[] args) throws Exception { testMyClassLoaderAndAppClassloader(); } }

 

執行結果如下,在上圖示紅行,會報錯,錯誤為轉型錯誤:

[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)

 

這裡可以看出來,不同類載入器載入的類,即使是同一個類,也是不相容的。因為這個例子中,一個是由Launcher$AppClassLoader載入,一個是自定義載入器載入的。

下面,我們將進一步驗證這個結論。

 

3、實驗2:自定義載入器 && 自定義載入器 (不同例項)

實驗 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);

    }

 

上圖紅色處,會報錯,報錯如下,原因是TestSample.class 預設在classpath下,由應用類載入器載入,而 instance 是由 classLoader 載入的,引數型別因此不匹配:

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)

 

實驗 3-2:

(改動僅標紅處)

 

    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);

    }

 

可以正常執行,結果為:

[Loaded TestSample from __JVM_DefineClass__]
MyClassLoader@41a4555e

 

實驗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);

    }

 

報錯,錯誤和實驗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)

 

實驗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);

    }

 

此時報錯和前面不同:

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,做了這麼多實驗,想必大概都瞭解了吧,引數型別不只是完全限定類名要一致,而且還需要類載入器一致才行。

簡單的引數傳遞,實際上隱藏瞭如此之多的東西。引數要傳對,看來不能拼人品啊,還是得靠知識。

 

三、關於Tomcat 中類載入器的思考

不知道看完了上面的實驗,大家有沒有想到一個問題,在我們的servlet 開發中,servlet-api.jar 包預設是由 tomcat 提供的,意思也就是,servlet-api.jar中的類應該都是由 tomcat 的common 類載入器載入的。(這個早已驗證,可翻我之前的部落格)

servlet-api.jar包中,有很多類,大家肯定用過 javax.servlet.Filter#doFilter :

public void doFilter(ServletRequest request, ServletResponse response,
                         FilterChain chain)
            throws IOException, ServletException;

 

我們思考一個問題,假如 我們在我們 web-inf/lib下,自己放上一個 servlet-api.jar,那麼載入 web-inf/lib 的自然就是 webappClassloader,那麼載入我們的filter的,也就是 webappClassloader。那麼我們的filter的引數,預設就應該只接受 webappclassloader 載入的 ServletRequest 、ServletResponse 類。

但是,很顯然,因為 Tomcat 的lib下面也有 servlet-api.jar,給我們的filter 傳遞的 reqeust引數,應該是由其 自己的common 類載入器載入的,問題來了,這樣還能呼叫成功我們的 filter 方法嗎?按理說,不可能,應該會報一個引數型別不匹配的錯誤才對,因為上一章的實驗結果就擺在那裡。

 

那就再測試一次吧,事實勝於雄辯,首先,我們將複用第一章的例子的servlet,唯一要改的,只是pom.xml(註釋了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,我們啟動Tomcat時,可以在catalina.sh/bat 中加一個引數:-XX:+TraceClassLoading,啟動後,訪問我們的 MyServlet,並沒有什麼異常(大家可以試試)。

然後我看了下,servletRequest等class,到底從哪載入的,下圖可以看出來,都是來自 tomcat 自身的 servlet-api.jar包:

 

而我們的 web-inf下的 servlet-api 包,完全就是個悲劇,被忽略了啊。。。慘。。。(我要你有何用??)

 

而且,另外一個層面來說,執行完全沒報錯,說明 webapp 中 載入servlet-api.jar包的classloader 和 tomcat 載入 servlet-api.jar包的classloader 為同一個,不然早就報錯了。那麼意思就是說, webapp 中載入 servlet-api.jar ,其實用的 tomcat 的common 類載入器去載入。(我真的柯南附體了。。。) 反證法也可以說明這一點,因為我們在 webapp的lib 下,是可以不放 servlet-api.jar包的,jar包只在 tomcat 有,而 webapp 的類載入器又不能去載入 tomcat 的東西,所以,只能說: webapp 類載入器委託了 tomcat 幫他載入。

 

我們可以看看 webappclassloader 的實現,我本地原始碼版本是 tomcat 7的,不過無所謂,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   // 檢查本載入器是否載入過了,本地有個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  // 呼叫了本載入器的本地方法,檢視是否載入過了
 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  // 先交給 擴充套件類載入器,免得把 jre/ext下面的類自己載入了出大事
 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  這個不管,我們這邊是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);  //預設為false,可以配置,如果為true,表示應該交給 tomcat 的common類載入器先載入
 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類載入器 載入失敗,則有自己載入
 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  // 如果自己載入失敗了,別說了,都甩給 tomcat 的common類載入器吧
 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             }

 

簡單歸納下:

1、webappclassloader 載入時,先看本載入器的快取,看看是否載入過了,載入過了直接返回,否則進入2;

2、先給 jdk 的jre/ext 類載入器載入, jre/ext 如果載入不了,會丟給 Bootstrap 載入器,如果載入到了,則返回,否則進入3;

3、判斷delegate 屬性,如果為true,則進入3.1,為false,則進入 3.2

    3.1  丟給tomcat 的common 類載入器,載入成功則返回,否則本載入器真正嘗試載入,成功則返回,否則拋異常:載入失敗。

    3.2 先讓自己類載入器嘗試,成功則返回,否則丟給 tomcat 載入,成功則返回,否則拋異常:載入失敗。

 

 

四、總結

物件,由類生成,類,由類載入器載入而來。 物件的方法引數的型別,也和類載入器息息相關, 這個引數是 類載入器 A 載入的class B型別,你必須也傳一個這樣的給我,我才認啊。

舉個例子,假設你先後有過兩個女朋友,前女友給你送了個iphone 8,現女友也送了你一個iphone 8, 這兩個iphone 8 都是同一個地方買的,那這兩個iPhone 8 能一樣嗎?要不問問你現女友去?

 

所以說啊,java這東西,他麼的易學難精。。。繼續努力吧。 下篇可以寫寫熱部署、OSGI的問題,(半桶水,我自己也要去研究下,哈哈)。。

&n