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