1、源码下载：http://archive.apache.org/dist/tomcat/tomcat-9/v9.0.30/src/

2、创建项目根目录：apache-tomcat-9-debug

3、创建子目录apache-tomcat-9-debug\apache-tomcat-9.0.30_src将tomcat源码解压到apache-tomcat-9.0.30_src文件夹下

4、创建子目录apache-tomcat-9-debug\catalina-home将apache-tomcat-9.0.30_src文件夹下的conf文件夹拷贝到catalina-home目录

5、创建外层pom.xml文件；

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
 
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.yang</groupId>
    <artifactId>apache-tomcat-study</artifactId>
    <name>Tomcat 9.0 Study</name>
    <version>1.0</version>
    <packaging>pom</packaging>
 
    <modules>
        <module>apache-tomcat-9.0.30_src</module>
    </modules>
</project>

目录结构如下

6、创建tomcat9依赖文件pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>org.apache.tomcat</groupId>
    <artifactId>tomcat9</artifactId>
    <name>tomcat9</name>
    <version>9.0</version>

    <build>
        <finalName>apache-tomcat-9.0.0.M26-src</finalName>
        <sourceDirectory>java</sourceDirectory>
        <!--<testSourceDirectory>test</testSourceDirectory>  test 下的有些文件报错，因此将test文件夹去掉了-->
        <resources>
            <resource>
                <directory>java</directory>
            </resource>
        </resources>
        <testResources>
            <testResource>
                <directory>test</directory>
            </testResource>
        </testResources>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.1</version>
                <configuration>
                    <encoding>UTF-8</encoding>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jar-plugin</artifactId>
                <version>2.6</version>
            </plugin>
        </plugins>
    </build>
    <dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.11</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>ant</groupId>
            <artifactId>ant</artifactId>
            <version>1.6.5</version>
        </dependency>
        <dependency>
            <groupId>org.easymock</groupId>
            <artifactId>easymock</artifactId>
            <version>3.4</version>
        </dependency>
        <dependency>
            <groupId>com.sun.xml.messaging.saaj</groupId>
            <artifactId>saaj-impl</artifactId>
            <version>1.3</version>
        </dependency>
        <dependency>
            <groupId>wsdl4j</groupId>
            <artifactId>wsdl4j</artifactId>
            <version>1.6.2</version>
        </dependency>
        <dependency>
            <groupId>javax.xml</groupId>
            <artifactId>jaxrpc</artifactId>
            <version>1.1</version>
        </dependency>
        <dependency>
            <groupId>org.eclipse.jdt.core.compiler</groupId>
            <artifactId>ecj</artifactId>
            <version>4.6.1</version>
        </dependency>
        <dependency>
            <groupId>javax.xml</groupId>
            <artifactId>jaxrpc-api</artifactId>
            <version>1.1</version>
        </dependency>
        <dependency>
            <groupId>javax.xml</groupId>
            <artifactId>jaxrpc-api</artifactId>
            <version>1.1</version>
        </dependency>
    </dependencies>
</project>

7、启动idea，选择"打开"，打开项目apache-tomcat-9-debug

8、右键点击apache-tomcat-9.0.30_src目录下java文件夹，选择Mark Directory As Source Root

9、添加配置文件，选择application；

main class配置：org.apache.catalina.startup.Bootstrap

vm option 配置：-Dfile.encoding=UTF-8 -Dcatalina.home=catalina-home -Dcatalina.base=catalina-home -Djava.endorsed.dirs=catalina-home/endorsed -Djava.io.tmpdir=catalina-home/temp -Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager -Djava.util.logging.config.file=catalina-home/conf/logging.properties

10、编译项目，可能会报错
Error:(320, 45) java: 找不到符号
符号: 变量 VERSION_9
位置: 类 org.eclipse.jdt.internal.compiler.impl.CompilerOptions
统一将错误改为VERSION_1_8即可
11、运行项目

启动、运行成功；
12、为何会有乱码：
原因：一般是中文编码不匹配导致；经跟踪，发现是ResourceBundle读取org\apache\catalina\startup\LocalStrings_zh_CN.properties文件时没有用utf8解码导致；
解决：修改org.apache.tomcat.util.res.StringManager类中的getString函数；
由
if (bundle != null) {
str = bundle.getString(key);
}
改为
if (bundle != null) {
str = new String(bundle.getString(key).getBytes(“ISO-8859-1”), “UTF-8”);
}
再次运行，一切正常