【原創】Java基礎之簡單修改jar包中的class

有時需要修改很多jar（假設這些jar都位於lib目錄）中其中一個jar中的某一個類，而且又沒有原始代碼或ide，這時最簡單的方式是：

1 進入lib目錄

# cd lib
# ls
test.jar dependency1.jar dependency2.jar

2 查看待修改jar包內類結構

$ unzip -l test.jar

or

$ vim test.jar

3 解壓待修改jar包

$ mkdir test_dir
$ unzip test.jar -d test_dir

4 查看class的jdk版本

$ vi -b test_dir/package/Class.class
:%!xxd

0000000: cafe babe 0000 0032 0123 0700 0201 0023 .......2.#.....#

取第15-16位，上例即32，

5 將要修改的文件放到class同目錄下（如果沒有源代碼可以嘗試反編譯），並修改

$ vi test_dir/package/Class.java

6 收集classpath

由於jar包可能非常多，所以使用命令來生成

$ ls *.jar|sed ':a;N;$!ba;s/\n/:/g'
test.jar:dependency1.jar:dependency2.jar

注linux下是:分隔，windows下是;分隔

7 編譯

$ javac -help
Usage: javac <options> <source files>
where possible options include:
  -g                         Generate all debugging info
  -g:none                    Generate no debugging info
  -g:{lines,vars,source}     Generate only some debugging info
  -nowarn                    Generate no warnings
  -verbose                   Output messages about what the compiler is doing
  -deprecation               Output source locations where deprecated APIs are used
  -classpath <path>          Specify where to find user class files and annotation processors
  -cp <path>                 Specify where to find user class files and annotation processors
  -sourcepath <path>         Specify where to find input source files
  -bootclasspath <path>      Override location of bootstrap class files
  -extdirs <dirs>            Override location of installed extensions
  -endorseddirs <dirs>       Override location of endorsed standards path
  -proc:{none,only}          Control whether annotation processing and/or compilation is done.
  -processor <class1>[,<class2>,<class3>...] Names of the annotation processors to run; bypasses default discovery process
  -processorpath <path>      Specify where to find annotation processors
  -parameters                Generate metadata for reflection on method parameters
  -d <directory>             Specify where to place generated class files
  -s <directory>             Specify where to place generated source files
  -h <directory>             Specify where to place generated native header files
  -implicit:{none,class}     Specify whether or not to generate class files for implicitly referenced files
  -encoding <encoding>       Specify character encoding used by source files
  -source <release>          Provide source compatibility with specified release
  -target <release>          Generate class files for specific VM version
  -profile <profile>         Check that API used is available in the specified profile
  -version                   Version information
  -help                      Print a synopsis of standard options
  -Akey[=value]              Options to pass to annotation processors
  -X                         Print a synopsis of nonstandard options
  -J<flag>                   Pass <flag> directly to the runtime system
  -Werror                    Terminate compilation if warnings occur
  @<filename>                Read options and filenames from file

實際命令

$ javac -cp test.jar:dependency1.jar:dependency2.jar -source 1.6 -target 1.6 -sourcepath test_dir -d test_dir test_dir/package/Class.java

8 重新打jar包

$ cd test_dir
$ zip -r test.jar ./*

9 刪除臨時目錄和原始jar

$ cd ..
$ cp test_dir/test.jar test.jar
$ /bin/rm -rf test_dir

10 測試修改生效