sbt 项目通过 sbt-aspectj-plugin 使用 AspectJ

calendar Nov 17, 2015   ---   · 3 min read · AspectJ sbt  ·
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Java 在运用面向方向编程时，依照  AspectJ 的语法自己书写 *.aj 文件可以得到尽可能大的控制能力。如果是一个 sbt 的项目，有一个 sbt-aspectj-plugin 插件可以帮上我们的忙。那么如何应用这个插件呢？ 该插件首页面告诉我们要在 project/plugins.sbt 中加上下面这句话

1addSbtPlugin("com.typesafe.sbt" % "sbt-aspectj" % "0.10.4")
其他就是参考例子 runnable sample projects，然而这几个例子并非那么直白。所以还是自己做一个最简单的例子来体验 sbt 项目如何使用 AspectJ.

这是一个默认的 sbt+AspectJ 的项目布局
 1sbt-aspectj-sample/
 2├── build.sbt
 3├── project
 4│   ├── build.properties
 5│   └── plugins.sbt
 6└── src
 7    └── main
 8        ├── aspectj
 9        │   └── SampleAspect.aj
10        └── java
11            └── sample
12                └── Sample.java
各文件的内容分别如下：

build.sbt
 1import com.typesafe.sbt.SbtAspectj._
 2import com.typesafe.sbt.SbtAspectj.AspectjKeys._
 3
 4name := "sbt-aspectj-sample"
 5
 6val main = (project in file("."))
 7    .settings(aspectjSettings: _*)
 8    .settings(
 9        inputs in Aspectj <+= compiledClasses,
10        products in Compile <<= products in Aspectj,
11        products in Runtime <<= products in Compile,
12        verbose in Aspectj := true
13    )

上面的配置是把属性用 project.setting() 方法来设置，也可以不依赖于 (project in file("."))，下面的配置是一样的
 1import com.typesafe.sbt.SbtAspectj._
 2import com.typesafe.sbt.SbtAspectj.AspectjKeys._
 3
 4name := "sbt-aspectj-sample"
 5
 6aspectjSettings
 7
 8inputs in Aspectj <+= compiledClasses
 9
10products in Compile <<= products in Aspectj
11
12products in Runtime <<= products in Compile

和 .settings(aspectjSetting:_*) 一样，上面的 aspectjSetting 同样是关键，否则会报如下错误：
1[error] References to undefined settings:
2[error]
3[error]   aspectj:inputs from aspectj:inputs (/Users/yanbin/Desktop/sbt-aspectj-sample/build.sbt:8)
4[error]
5[error]   aspectj:products from compile:products (/Users/yanbin/Desktop/sbt-aspectj-sample/build.sbt:10)

build.properties (只放了一行)
1sbt.version=0.13.5

plugins.sbt (加载 sbt-aspectj 依赖)
1addSbtPlugin("com.typesafe.sbt" % "sbt-aspectj" % "0.10.4")

src/main/java/sample/Sample.java (Java 代码在 sbt 项目中默认位置)
1package sample;
2
3public class Sample {
4    public static void main(String[] args) {
5        System.out.println("Print from Sample class");
6    }
7}

src/main/aspectj/SampleAspect.aj (方面定义文件默认放在 src/main/aspectj 目录中)
1public aspect SampleAspect {
2
3  before(): execution(* sample.Sample.main(..)) {
4    System.out.println("Weaved content from SampleAspect");
5  }
6}

用 aspect 定义方面，文件名是  *.aj，它也可以用 Java 一样用  package sample 来定义包名，但没要求包名与目录层次一致。上面方面定义的是在招待 sample.Sample.main() 方法前打印 "Wearved content from SampleAspect".

现在来看执行效果

进到 sbt-aspectj-sample 目录中，然后执行 sbt 命令，接着执行 sbt 的 run 任务就会看到下面的输出
 1[I] ➜  sbt-aspectj-sample git:(master) ✗ sbt
 2[info] Loading project definition from /Users/yanbin/Desktop/sbt-aspectj-sample/project
 3[info] Set current project to sbt-aspectj-sample (in build file:/Users/yanbin/Desktop/sbt-aspectj-sample/)
 4> run
 5[info] Weaving 1 input with 1 AspectJ source to /Users/yanbin/Desktop/sbt-aspectj-sample/target/scala-2.10/aspectj/classes...
 6[info] info directory classpath entry does not exist: /Library/Java/JavaVirtualMachines/jdk1.8.0_45.jdk/Contents/Home/jre/lib/sunrsasign.jar
 7[info] info zipfile classpath entry does not exist: /Library/Java/JavaVirtualMachines/jdk1.8.0_45.jdk/Contents/Home/jre/classes
 8[info] info Pipelining compilation
 9[info] info compiling /Users/yanbin/Desktop/sbt-aspectj-sample/src/main/aspectj/SampleAspect.aj
10[info] info weaver operating in reweavable mode.  Need to verify any required types exist.
11[info] info woven aspect SampleAspect (from /Users/yanbin/Desktop/sbt-aspectj-sample/src/main/aspectj/SampleAspect.aj)
12[info] info woven class sample.Sample (from /Users/yanbin/Desktop/sbt-aspectj-sample/target/scala-2.10/classes/sample/Sample.class)
13[info] info Compiler took 768ms
14[info] Running sample.Sample
15Weaved content from SampleAspect
16Print from Sample class
17[success] Total time: 1 s, completed Nov 17, 2015 11:15:09 PM

从上面可看出在执行 sample.Sample.main() 方法之前执行了方面 SampleAspect 中 before() 指定的方法，说明拦截成功。

这里只是一个最简单的例子实现了方面的正确切入，更多 AspectJ 的语法如 after(), around() 或捕获异常等，请参考其他相关资料。

 比如可以把 src/main/aspectj/SampleAspect.aj 改为
 1package sample;<br/><br/>
 2privileged public aspect SampleAspect {
 3
 4  pointcut aroundMain(String[] args) : execution(* sample.Sample.main(..)) && args(args);
 5
 6  void around(String[] args): aroundMain(args) {
 7    System.out.println("inputs: " + java.util.Arrays.asList(args));
 8    proceed(args);
 9    System.out.println("after method");
10  }
11}

同时把 build.sbt 中的

verbose in Aspectj := true

改为

verbose in Aspectj := false

再进到 sbt 控制台执行下 run param1 param2 看下输出是
 1[I] ➜  sbt-aspectj-sample git:(master) ✗ sbt
 2[info] Loading project definition from /Users/yanbin/Desktop/sbt-aspectj-sample/project
 3[info] Set current project to sbt-aspectj-sample (in build file:/Users/yanbin/Desktop/sbt-aspectj-sample/)
 4> run param1 param2
 5[info] Running sample.Sample param1 param2
 6inputs: [param1, param2]
 7Print from Sample class
 8after method
 9[success] Total time: 0 s, completed Nov 17, 2015 11:45:00 PM
10>

这样拦截了整个方法的前前后后的执行，借此可以修改目标方法原有的参数值。 永久链接 https://yanbin.blog/sbt-project-sbt-aspectj-plugin-use-aspectj/, 来自 隔叶黄莺 Yanbin's Blog
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