Tomcat 6 --- 使用Jasper引擎解析JSP

熟悉JAVA web开发的朋友都知道JSP会被转换成java文件（预编译），然后编译成class使用，即按照JSP-->java-->class的过程进行编译。

由于JVM只认识class文件，它不知道什么是JSP，因此在tomcat中 如何把JSP解析成java文件 就是本文所要描述的问题。

其他翻译内容参考：Tomcat官方文档翻译

如有错误，请予指正。

什么是Jasper

　　Jasper是tomcat中使用的JSP引擎，在Tomcat 6中使用的是Jasper 2，相对于原来的版本作了不少的改进，比如：JSP的标签缓冲池、后台编译、页面改变时自动重新编译、Eclipse中JDT编译等等。

　　那么Jasper到底是做什么的呢？

　　简单的说，就是把JVM不认识的JSP文件解析成java文件，然后编译成class文件提供使用。目前有很多的JSP解析引擎，Tomcat中使用的是Jasper。

　　在Tomcat中可以通过配置 CATALINA_HOME/conf/web.xml 中的内容，配置Jasper的选项（web.xml中的内容很长，截取其中的一部分）：

  <!-- The JSP page compiler and execution servlet, which is the mechanism  -->
  <!-- used by Tomcat to support JSP pages.  Traditionally, this servlet    -->
  <!-- is mapped to the URL pattern "*.jsp".  This servlet supports the     -->
  <!-- following initialization parameters (default values are in square    -->
  <!-- brackets):                                                           -->
  <!--                                                                      -->
  <!--   checkInterval       If development is false and checkInterval is   -->
  <!--                       greater than zero, background compilations are -->
  <!--                       enabled. checkInterval is the time in seconds  -->
  <!--                       between checks to see if a JSP page (and its   -->
  <!--                       dependent files) needs to  be recompiled. [0]  -->
  <!--                                                                      -->
  <!--   classdebuginfo      Should the class file be compiled with         -->
  <!--                       debugging information?  [true]                 -->
  <!--                                                                      -->
  <!--   classpath           What class path should I use while compiling   -->
  <!--                       generated servlets?  [Created dynamically      -->
  <!--                       based on the current web application]          -->
  <!--                                                                      -->
  <!--   compiler            Which compiler Ant should use to compile JSP   -->
  <!--                       pages.  See the jasper documentation for more  -->
  <!--                       information.                                   -->
  <!--                                                                      -->
  <!--   compilerSourceVM    Compiler source VM. [1.5]                      -->
  <!--                                                                      -->
  <!--   compilerTargetVM    Compiler target VM. [1.5]                      -->
  <!--                                                                      -->
  <!--   development         Is Jasper used in development mode? If true,   -->
  <!--                       the frequency at which JSPs are checked for    -->
  <!--                       modification may be specified via the          -->
  <!--                       modificationTestInterval parameter. [true]     -->
  <!--                                                                      -->
  <!--   displaySourceFragment                                              -->
  <!--                       Should a source fragment be included in        -->
  <!--                       exception messages? [true]                     -->
  <!--                                                                      -->
  <!--   dumpSmap            Should the SMAP info for JSR45 debugging be    -->
  <!--                       dumped to a file? [false]                      -->
  <!--                       False if suppressSmap is true                  -->
  <!--                                                                      -->
  <!--   enablePooling       Determines whether tag handler pooling is      -->
  <!--                       enabled. This is a compilation option. It will -->
  <!--                       not alter the behaviour of JSPs that have      -->
  <!--                       already been compiled. [true]                  -->
  <!--                                                                      -->
  <!--   engineOptionsClass  Allows specifying the Options class used to    -->
  <!--                       configure Jasper. If not present, the default  -->
  <!--                       EmbeddedServletOptions will be used.           -->
  <!--                                                                      -->
  <!--   errorOnUseBeanInvalidClassAttribute                                -->
  <!--                       Should Jasper issue an error when the value of -->
  <!--                       the class attribute in an useBean action is    -->
  <!--                       not a valid bean class?  [true]                -->
  <!--                                                                      -->
  <!--   fork                Tell Ant to fork compiles of JSP pages so that -->
  <!--                       a separate JVM is used for JSP page compiles   -->
  <!--                       from the one Tomcat is running in. [true]      -->
  <!--                                                                      -->
  <!--   genStringAsCharArray                                               -->
  <!--                       Should text strings be generated as char       -->
  <!--                       arrays, to improve performance in some cases?  -->
  <!--                       [false]                                        -->
  <!--                                                                      -->
  <!--   ieClassId           The class-id value to be sent to Internet      -->
  <!--                       Explorer when using <jsp:plugin> tags.         -->
  <!--                       [clsid:8AD9C840-044E-11D1-B3E9-00805F499D93]   -->
  <!--                                                                      -->
  <!--   javaEncoding        Java file encoding to use for generating java  -->
  <!--                       source files. [UTF8]                           -->
  <!--                                                                      -->
  <!--   keepgenerated       Should we keep the generated Java source code  -->
  <!--                       for each page instead of deleting it? [true]   -->
  <!--                                                                      -->
  <!--   mappedfile          Should we generate static content with one     -->
  <!--                       print statement per input line, to ease        -->
  <!--                       debugging?  [true]                             -->
  <!--                                                                      -->
  <!--   modificationTestInterval                                           -->
  <!--                       Causes a JSP (and its dependent files) to not  -->
  <!--                       be checked for modification during the         -->
  <!--                       specified time interval (in seconds) from the  -->
  <!--                       last time the JSP was checked for              -->
  <!--                       modification. A value of 0 will cause the JSP  -->
  <!--                       to be checked on every access.                 -->
  <!--                       Used in development mode only. [4]             -->
  <!--                                                                      -->
  <!--   recompileOnFail     If a JSP compilation fails should the          -->
  <!--                       modificationTestInterval be ignored and the    -->
  <!--                       next access trigger a re-compilation attempt?  -->
  <!--                       Used in development mode only and is disabled  -->
  <!--                       by default as compilation may be expensive and -->
  <!--                       could lead to excessive resource usage.        -->
  <!--                       [false]                                        -->
  <!--                                                                      -->
  <!--   scratchdir          What scratch directory should we use when      -->
  <!--                       compiling JSP pages?  [default work directory  -->
  <!--                       for the current web application]               -->
  <!--                                                                      -->
  <!--   suppressSmap        Should the generation of SMAP info for JSR45   -->
  <!--                       debugging be suppressed?  [false]              -->
  <!--                                                                      -->
  <!--   trimSpaces          Should white spaces in template text between   -->
  <!--                       actions or directives be trimmed?  [false]     -->
  <!--                                                                      -->
  <!--   xpoweredBy          Determines whether X-Powered-By response       -->
  <!--                       header is added by generated servlet  [false]  -->
  <!--                                                                      -->
  <!-- If you wish to use Jikes to compile JSP pages:                       -->
  <!--   Please see the "Using Jikes" section of the Jasper-HowTo           -->
  <!--   page in the Tomcat documentation.                                  -->

    <servlet>
        <servlet-name>jsp</servlet-name>
        <servlet-class>org.apache.jasper.servlet.JspServlet</servlet-class>
        <init-param>
            <param-name>fork</param-name>
            <param-value>false</param-value>
        </init-param>
        <init-param>
            <param-name>xpoweredBy</param-name>
            <param-value>false</param-value>
        </init-param>
        <load-on-startup>3</load-on-startup>
    </servlet>

　　具体的参数，上面都有解释，这里就不多赘述了。

　　都是些调节JSP编译的参数，比如多长时间检测一次，debug的调试信息相关配置，编译信息等等。

如何利用Japser.Jspc自定义预编译JSP

　　1 jasper相关jar包

　　在tomcat6中提供了几个jasper的jar包，相对于之前版本，去掉了jasper-compiler.jar以及jasper-runtime.jar，合并为jasper.jar

　　另外如果开发者自己想要编译JSP，还需要使用ant以及tomcat-juli.jar。

　　2 ant相关jar包

　　其中ant相关的jar包可以去官网下载ant.zip，然后解压提取其中lib内的jar包。

　　3 tomcat-juli相关jar包

　　tomcat-juli.jar位于CATALINA_HOME/bin/目录下。

　　在Eclipse的构建路径下添加上述相关的jar包即可，然后创建测试类:

　　添加JAR包步骤：右键工程-->Properties-->Java Build Path-->Libraries-->Add External JARs-->选择添加的JAR包-->OK

package com.test;
import org.apache.jasper.JspC;

public class testCompiler{
    public String jspcTest(){
        String error="";
        try {
            JspC jspc = new JspC();
            //第一种方式
            String[] arg0 = {"-uriroot", "F:/apache-tomcat-6.0.43/webapps/ROOT", "-d", "F:/test",
                    "index.jsp" };
            jspc.setArgs(arg0); 

            //第二种方式
            /*jspc.setUriroot("F:/apache-tomcat-6.0.43/webapps/ROOT");//web应用的root目录
            jspc.setOutputDir("F:/test");//.java文件和.class文件的输出目录
            jspc.setJspFiles("index.jsp");//要编译的jsp  */

            jspc.setCompile(true);//是否编译 false或不指定的话只生成.java文件
            jspc.execute();
        }catch(Exception e){
            error=e.toString();
        }
        return error;
    }
    public static void main(String args[]){
        testCompiler t=new testCompiler();
        System.out.println(t.jspcTest());
    }
}  

　　可以使用两种方式进行自定义的JSP编译。

　　测试后，可以在 F:/test 目录下发现编译出的index.jsp的java文件以及class文件。

参考

【1】Jasper2 JSP引擎：http://tomcat.apache.org/tomcat-6.0-doc/jasper-howto.html

【2】解读JSP解析过程：http://www.cnblogs.com/zollty/p/3309310.html

【3】使用Jspc编译JSP文件：http://kjah.iteye.com/blog/625588