Tomcat 6 --- 使用Jasper引擎解析JSP
熟悉JAVA web開發的朋友都知道JSP會被轉換成java文件（預編譯），然后編譯成class使用，即按照JSP-->java-->class的過程進行編譯。
由于JVM只認識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問題。
其他翻譯內容參考：Tomcat官方文檔翻譯
如有錯誤，請予指正。
什么是Jasper
Jasper是tomcat中使用的JSP引擎，在Tomcat 6中使用的是Jasper 2，相對于原來的版本作了不少的改進，比如： JSP的標簽緩沖池、后臺編譯、頁面改變時自動重新編譯、Eclipse中JDT編譯 等等。
那么Jasper到底是做什么的呢？
簡單的說，就是把JVM不認識的JSP文件解析成java文件，然后編譯成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中可以通過配置 CATALINA_HOME/conf/web.xml 中的內容，配置Jasper的選項（web.xml中的內容很長，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>具體的參數，上面都有解釋，這里就不多贅述了。
都是些調節JSP編譯的參數，比如多長時間檢測一次，debug的調試信息相關配置，編譯信息等等。
如何利用Japser.Jspc自定義預編譯JSP
1 jasper相關jar包
在tomcat6中提供了幾個jasper的jar包，相對于之前版本，去掉了jasper-compiler.jar以及jasper-runtime.jar，合并為jasper.jar
另外如果開發者自己想要編譯JSP，還需要使用ant以及tomcat-juli.jar。
2 ant相關jar包
其中ant相關的jar包可以去官網下載ant.zip，然后解壓提取其中lib內的jar包。
3 tomcat-juli相關jar包
tomcat-juli.jar位于CATALINA_HOME/bin/目錄下。
在Eclipse的構建路徑下添加上述相關的jar包即可，然后創建測試類:
添加JAR包步驟：右鍵工程-->Properties-->Java Build Path-->Libraries-->Add External JARs-->選擇添加的JAR包-->OK
package com.test;
import org.apache.jasper.JspC;
public class testCompiler{
public String jspcTest(){
String error="";
try {
JspC jspc = new JspC();
//第一種方式
String[] arg0 = {"-uriroot", "F:/apache-tomcat-6.0.43/webapps/ROOT", "-d", "F:/test",
"index.jsp" };
jspc.setArgs(arg0);
//第二種方式
/*jspc.setUriroot("F:/apache-tomcat-6.0.43/webapps/ROOT");//web應用的root目錄
jspc.setOutputDir("F:/test");//.java文件和.class文件的輸出目錄
jspc.setJspFiles("index.jsp");//要編譯的jsp */
jspc.setCompile(true);//是否編譯 false或不指定的話只生成.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());
}
} 可以使用兩種方式進行自定義的JSP編譯。
測試后，可以在 F:/test 目錄下發現編譯出的index.jsp的java文件以及class文件。
參考
【1】Jasper2 JSP引擎： http://tomcat.apache.org/tomcat-6.0-doc/jasper-howto.html
【2】解讀JSP解析過程： http://www.cnblogs.com/zollty/p/3309310.html
【3】使用Jspc編譯JSP文件： http://kjah.iteye.com/blog/625588
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