Tomcat 8

JDTCompiler.java

/**
 * Compile the jsp file from the current engine context. As an side- effect,
 * tag files that are referenced by this page are also compiled.
 * 编译 jsp 文件，对应生成 java 文件和 class 文件
 * @param compileClass
 *            If true, generate both .java and .class file If false,
 *            generate only .java file
 */
void org.apache.jasper.compiler.Compiler.compile(boolean compileClass)

/**
 * Compile the jsp file into equivalent servlet in .java file
 * 编译 jsp 文件，生成相应的 servlet 的 java文件
 * @return a smap for the current JSP page, if one is generated, null
 *         otherwise
 */
String[] org.apache.jasper.compiler.Compiler.generateJava()

/**
 * Compile the servlet from .java file to .class file
 * 将 servlet 的 java 文件编译成 class 文件
 */
void org.apache.jasper.compiler.JDTCompiler.generateClass(String[] smap)

JspCompilationContext.java

/**
 * Full path name of the Java file into which the servlet is being
 * generated.
 * jsp 文件生成对应的 servlet java文件的全路径
 */
public String getServletJavaFileName() {
    if (servletJavaFileName == null) {
        servletJavaFileName = getOutputDir() + getServletClassName() + ".java";
    }
    return servletJavaFileName;
}

SmapUtil.java  写出 jsp 文件对应的 servlet 的 class 文件

public static void installSmap(String[] smap)
    throws IOException {
    if (smap == null) {
        return;
    }

    for (int i = 0; i < smap.length; i += 2) {
        File outServlet = new File(smap[i]);
        SDEInstaller.install(outServlet,
                smap[i+1].getBytes(StandardCharsets.ISO_8859_1));
    }
}

参考：

http://www.cnblogs.com/mapeng-cnblogs/p/4226005.html
https://my.oschina.net/heroShane/blog/198450
https://www.ibm.com/developerworks/cn/java/j-lo-servlet/