Quality is a key element in measuring the success of a piece of software. For commercial software systems, especially enterprise application software systems, in addition to software running quality and document quality, code quality is also very important. When software development is in the coding stage, the biggest risk is how to ensure the readability and consistency of the code, so that the cost of software maintenance will not be very high.
In the process of software development, the following situations can be seen everywhere:
1) The software maintenance time is long, and the enthusiasm of the maintainers is not high:
Developers who have done software maintenance, especially when taking over the source code of products that they did not develop themselves, are still "amazing" at lengthy, uncommented passages in the code, even with good documentation. It is so difficult to understand, let alone modify or add new functions. Therefore, many developers are reluctant to do software maintenance work.
2) New developers take a long time to integrate into the team:
In addition to the lack of effective mechanisms such as good training and documentation, each person's set of coding styles can easily lead to insufficient understanding of existing code by new members, or even deviations.
improve The quality of the code , in addition to improving the logical In addition to control and understanding of business processes, there are also improvements in the code itself. Space , for example some potential problems can be avoided very early. Similar to what's on the coding specification, if it's all up to the coders Do a self-check, then no doubt Requires a lot of work, if static checking of the code is available If the tool is checked, then the efficiency of coding will be greatly improved.
The current code inspection work of the project team is basically done manually, which is difficult to implement, and the inspection effect is not very obvious. PMD is such a tool, which can directly use its own rules (of course, you can also use your own rules) to analyze the Java source program to find out the problems of the program, which can greatly reduce the cumbersome code inspection work. , play a guiding role for the future maintenance and development work of the project team.
This article mainly introduces how to use the pmd tool to automatically check the code to avoid some potential problems and find out the logic errors of the code.
2. Introduction to PMD
PMD is an open source tool for analyzing errors in Java code. Unlike other analysis tools, PMD learns code errors through static analysis. That is, to report errors without running the Java program. PMD comes with many rules that can be used directly, using these rules to find many problems in Java source programs, such as:
® Potential bug: empty try/catch/finally/switch statements
® Unused code: unused local variables, parameters, private methods, etc.
® Optional Code: Misuse of String/StringBuffer
® Complex expressions: unnecessary if statements, for loops that can be done with while loops
® Duplicated code: copying/pasting code means copying/pasting bugs
® The body of the loop creates a new object: try not to instantiate a new object in the body of a for or while loop
@ Resource close: Make sure to close Connect, Result, Statement, etc. after use
In addition, users can also define their own rules to check whether the Java code conforms to some specific coding standards. For example, you could write a rule that asks PMD to find out all operations that create Thread and Socket objects.
3. How it works
PMD的核心是JavaCC解析器生成器。PMD结合运用JavaCC和EBNF（扩展巴科斯-诺尔范式，Extended Backus-Naur Formal）语法，再加上JJTree，把Java源代码解析成抽象语法树（AST，Abstract Syntax Tree）。显然，这句话不那么好懂，且看下文具体说明。
从根本上看，Java源代码只是一些普通的文本。不过，为了让解析器承认 这些普通的文本是合法的Java代码，它们必须符合某种特定的结构要求。这种结构可以用一种称为EBNF的句法元语言表示，通常称为“语法” （Grammar）。JavaCC根据语法要求生成解析器，这个解析器就可以用于解析用Java编程语言编写的程序。
不过实际运行 中的PMD还要经过JJTree的一次转换。JJTree是一个JavaCC的插件，通过AST扩充JavaCC生成的解析器。AST是一个Java符号 流之上的语义层。有了JJTree，语法分析的结果不再是“System, ., out, ., . println”之类的符号序列，而是一个由对象构成的树型层次结构。例如，下面是一段简单的Java代码以及与之对应的AST。
Java源代码：
public class Foo {
public void bar() {
System.out.println("hello world");
}
}
对应的抽象语法树
CompilationUnit
TypeDeclaration
ClassDeclaration
UnmodifiedClassDeclaration
ClassBody
ClassBodyDeclaration
MethodDeclaration
ResultType
MethodDeclarator
FormalParameters
Block
BlockStatement
Statement
StatementEXPression
PrimaryExpression
PrimaryPrefix
Name
PrimarySuffix
Arguments
ArgumentList
Expression
PrimaryExpression
PrimaryPrefix
Literal
4. PMD 的安装和运行
4.1 安装并从命令行运行PMD
你可以从PMD的网站下载PMD的二进制版本，或下载带源代码的版本，下载得到的都是ZIP文件。假设你下载了二进制版本，先把它解压缩到任意一个 目录。接下来怎么做，就要看你准备怎么用它——最简单的，如果要在一个Java源代码目录中运行PMD，只需直接在命令行上运行下面的命令：
E:/SoftWare/pmd-bin-4.2.1/pmd-4.2.1/bin>java -jar ../lib/pmd-4.2.1.jar D:/ebsser
vice/ebsservice/src text rulesets/unusedcode.xml
输出结果类如：
D:/ebsservice/ebsservice/src/com/sinosoft/service/policy/ebs/SMPolicyInput.java:
51 Avoid unused private fields such as 'logger'.
D:/ebsservice/ebsservice/src/com/sinosoft/service/policy/ebs/SMPolicyShow.java:2
5 Avoid unused private fields such as 'logger'.
D:/ebsservice/ebsservice/src/com/sinosoft/service/policy/ebs/SMQueryPolicyByPoli
cyNo.java:32 Avoid unused local variables such as 'visaStatus'.
D:/ebsservice/ebsservice/src/com/sinosoft/service/policy/ebs/SMQueryPolicyByPoli
cyNo.java:44 Avoid unused local variables such as 'temp'.
D:/ebsservice/ebsservice/src/com/sinosoft/service/policy/ebs/erisk/ESMPolicyInpu
t.java:28 Avoid unused private fields such as 'logger'.
D:/ebsservice/ebsservice/src/com/sinosoft/service/policy/ebs/jrisk/JSMPolicyInpu
t.java:22 Avoid unused private fields such as 'logger'.
一些可以加载必须参数前面或者后面的可选参数如下：
-debug: 打印debug日志信息
-targetjdk: 指定目标源代码的版本- 1.3, 1.4, 1.5, 1.6 or 1.7;
默认是1.5
-cpus: 指定创建的线程数
-encoding: 指定PMD检查的代码的编码方式
-excludemarker: 指定PMD需要忽略的行的标记，默认为NOPMD
-shortnames: 在报告中显示缩短的文件名
-linkprefix: HTML源文件的路径，只是为了HTML显示
-lineprefix: 自定义的锚，用于影响源文件中的行，只是用于HTML显示
-minimumpriority: 规则的优先级限制，低于优先级的规则将不被使用
-nojava: 不检查java文件，默认是检查java文件
-jsp: 检查JSP/JSF文件，默认不检查
-reportfile: 将报告输出到文件，默认是打印在控制台
-benchmark: 输出一个基准清单，默认输出到控制台
-xslt: 覆盖默认的xslt
-auxclasspath: 指定源代码文件使用的类路径
例如在windows系统中，例子如下：
c:/> java -jar pmd-4.2.1.jar c:/my/source/code text unusedcode,imports -targetjd
k 1.5 -debug
c:/> java -jar pmd-4.2.1.jar c:/my/source/code xml basic,design -encoding UTF-8
c:/> java -jar pmd-4.2.1.jar c:/my/source/code html typeresolution -auxclasspath
commons-collections.jar;derby.jar
4.2 在Eclipse中 安装 PMD插件运行方式
PMD可以作为插件集成到很多 流行 的 IDE中，很多的插件中都包含了PMD的jar文件，这个jar文件中包含了规则集。所以虽然一些插件中使用 rulesets/unusedcode.xml来作为参数引用规则集，但是实际上是使用getResourceAsStream()方法来从PMD的 jar文件中加载。
由于Eclipse是比较流行的开源Java/J2EE开发IDE，所以本文主要介绍如何在Eclipse中使用PMD工具进行代码的检查。
4.2.1 安装基于Eclipse IDE的插件
安装Eclipse的PMD插件的过程如下：
® 启动Eclipse
® 选择Help-->Software Updates-->Find and Install
® 选择Next,选择New remote site
® 在Name框中输入PMD，URL框中输入http://pmd.sf.net/eclipse
® 在之后的对话框中一直点击下一步或者接受协议，完成Eclipse的PMD插件的安装
也可以通过下载 最新 的zip文件按，然后执行上述过程，只是使用New locale site来代替New remote site，并使用下载的zip文件。
可以通过Windows-->Preferences来配置PMD。
通过右键一个 项目 ， 然后选择PMD-->Check node with PMD，即可使用PMD工具检查代码。如果要进行重复代码检测，那么右键一个项目后，选择PMD-->Find suspect cut and paste。检查结果会放在reports目录下，文件名为cpd-report.txt。
可以通过使用Eclipse的帮助系统来查看PMD插件的文档。
在安装完更新后，如果发生了一个异常，例如”java.lang.RuntimeException: Could not find that class xxxx”，这时试着删除workspace中的.metadata/plugins/net.sourceforge.pmd.eclipse目录下的 ruleset.xml文件。
4.2.2 使用PMD
1、启动Eclipse IDE，打开工程，选择 "Windows"->"Preferences"下的PMD项，其中Rules Configuration 项目可以配置PMD的检查规则，自定义检查规则也可以在此通过Import的方式导入到PMD中
2、配置好后，鼠标右键点击工程中需要检查的JavaSource，选择"PMD"->"Check Code With PMD" ,之后PMD就会通过规则检查你的JavaSource了并且将信息显示在PMD自己的视图上
3、示例
import java.util.*;
public class Test {
public static void main(String[] args) {
try{
if(true) {}
System.out.println("Hello World!");
} catch(Exception e) {
}
}
}
以上代码PMD会检查出：catch块中没有内容、if判断块中没有内容、代码中出现System.out.println等警告描述
4.3 使用Ant进行调用
下面是主要的Ant配置信息
<path id="pmd.path">
<fileset dir="${lib.dir}/pmd-3.8">
<include name="**/*.jar" />
</fileset>
</path>
<taskdef name="pmd" classname="net.sourceforge.pmd.ant.PMDTask" classpathref="pmd.path"/>
<taskdef name="cpd" classname="net.sourceforge.pmd.cpd.CPDTask" classpathref="pmd.path"/>
<target name="pmd">
<pmd shortFilenames="true">
<ruleset>rulesets/favorites.xml</ruleset>
<formatter type="html" toFile="d:/foo.html" toConsole="false"/>
<fileset dir="${src.dir}">
<include name="**/*.java"/>
</fileset>
</pmd>
</target>
<target name="cpd">
<cpd minimumTokenCount="100" outputFile="d:/cpd.txt">
<fileset dir="${src.dir}">
<include name="**/*.java"/>
</fileset>
</cpd>
</target>
用Ant命令运行build.xml，PMD就会按照你设定好的规则自动执行代码检查了。
5. 关于PMD规则
选择合适的规则
运行所有的规则集中的规则会产生非常多的冲突，这些冲突中的很多是不重要的。在这么多的冲突中寻找你关心的部分结果就没有什么效率可言了。
所以需要从明显的规则集，也就是说必须要改的 地方 开始是比较好的一个选择，例如只是运行unusedcode检查，然后修改没有使用的局部变量和成员变量。然后运行基本的检查，修改所有的空语句，例如if语句等。最后可以执行与设计 相关的或者存在一定争议的规则集，或者自定义的规则集。
自带规则的介绍： （PMD插件分析代码规则（中文）.xls）
PMD 自带了很多规则集合，并且分类写入不同的 ruleset 文件，如
Basic 包含每人都必须遵守的代码最佳实践，如EmptyCatchBlock
Braces 关于条件分支的规则，如IfStmtsMustUseBraces
Code Size 关于代码大小的规则，如方法的长度，参数的长度，属性的个数等
Clone 克隆实现的规则，如是否有super.clone()
Controversial 一些有争议的规则，如UnnecessaryConstructor不必要的构造器
Coupling 对象连接有关的规则
Design 可以检查有问题的设计，如SwitchStmtsShouldHaveDefault
Finalizers 使用finalizers时需遵循的规则，如FinalizeOnlyCallsSuperFinalize
Import Statements 和import有关的规则，如DuplicateImports重复import
J2EE 唯一规则UseProperClassLoader，class.getClassLoader()可能不正确，用
Thread.currentThread().getContextClassLoader() 代替
Javabeans 和javabean规范有关的规则，有BeanMembersShouldSerialize属性必须
序列化和MissingSerialVersionUID缺少序列化ID
JUnit Tests 和JUnit测试有关的，如JUnitSpelling拼写检查等
Logging (Java) Check some wrong usage of Logger, such as MoreThanOneLogger multiple Logger
Logging (Jakarta) Use some rules of Jakarta Logger, with UseCorrectExceptionLogging
Improper handling of exceptions and whether ProperLogger defines Logger correctly
Migrating JDK version porting rules, such as ReplaceVectorWithList to replace Vector with List
Naming and naming related rules, names that are too short or too long, naming conventions, etc.
http://blog.csdn.net/sadamdiyi/article/details/6073694