Aspectj LTW 实践

Aspectj LTW 实践

  tech
  5 minutes read (About 788 words)

Aspectj LTW 实践

本文将介绍使用 AspectJ，介绍它的 3 种织入方式。

AspectJ 作为 AOP 编程的完全解决方案，提供了三种织入时机，分别为

  1. compile-time：编译期织入，在编译的时候一步到位，直接编译出包含织入代码的 .class 文件
  2. post-compile：编译后织入，增强已经编译出来的类，如我们要增强依赖的 jar 包中的某个类的某个方法
  3. load-time：在 JVM 进行类加载的时候进行织入

通过一个简单的应用程序，来说明Aspectj LTW 的简单实践

1.新建被织入的类 Simple.java

Simple.java
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public class Simple {
  public static void main(String[]argv) {
    countFast(1000);
    countSlow(1000);
  }

  public static void countSlow(int value) {
    count(value,5);
  }

  public static void countFast(int value) {
    count(value,0);
  }

  private static void count(int value, int delay) {
    for (int i=0;i<value;i++) {
      try {Thread.sleep(delay);} catch (Exception e) {}
    }
  }
}

2.aspectj 织入类WhereDoesTheTimeGo.aj

WhereDoesTheTimeGo.aj
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aspect WhereDoesTheTimeGo {

  pointcut methodsOfInterest(): execution(* *(..)) &&
                              !within(WhereDoesTheTimeGo);

  private int nesting = 0;

  Object around(): methodsOfInterest() {
    nesting++;
    long stime=System.currentTimeMillis();
    Object o = proceed();
    long etime=System.currentTimeMillis();
    nesting--;
    StringBuilder info = new StringBuilder();
    for (int i=0;i<nesting;i++) {
      info.append("  ");
    }
    info.append(thisJoinPoint+" took "+(etime-stime)+"ms");
    System.out.println(info.toString());
    return o;
  }
}

编译时织入

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ajc WhereDoesTheTimeGo.aj Simple.java
java Simple
//输出
    execution(void Simple.count(int, int)) took 2ms
  execution(void Simple.countFast(int)) took 4ms
    execution(void Simple.count(int, int)) took 5049ms
  execution(void Simple.countSlow(int)) took 5049ms
execution(void Simple.main(String[])) took 5054ms

运行时织入

首先清理 文件夹内所有编译好的class

这时候只编译 Simple.java 生成Simple.class

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javac Simple.java

这个Simple.class 只是简简单单的编译后class文件

通过aj编译期，将 WhereDoesTheTimeGo.aj 编译为 jar包

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ajc WhereDoesTheTimeGo.aj -outxml -outjar timing.jar

这个timing.jar 就是我们要织入的jar包，通过aj编辑器后，内部结构是这样的：

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jar -tvf timing.jar
  55 Wed Feb 25 22:06:30 GMT 2009 META-INF/MANIFEST.MF
5149 Wed Feb 25 22:06:32 GMT 2009 WhereDoesTheTimeGo.class
  85 Wed Feb 25 22:06:32 GMT 2009 META-INF/aop-ajc.xml

验证：

执行

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java Simple
//没有任何输出

配置-javaagent 

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java -javaagent:<pathToAspectj>/lib/aspectjweaver.jar
     -classpath "code;timing.jar;<pathToAspectj>/lib/aspectjrt.jar" Simple
//输出
   execution(void Simple.count(int, int)) took 1ms
 execution(void Simple.countFast(int)) took 6ms
   execution(void Simple.count(int, int)) took 5114ms
 execution(void Simple.countSlow(int)) took 5114ms
execution(void Simple.main(String[])) took 5121ms

注意：在linux 系统，classpath 的分隔符要改为 :

配置aop-ajc.xml

我们在生成织入jar时，是有aj编译期自动生成了默认的ajc配置文件，其内容是：

aop-ajc.xml
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<aspectj> 
<aspects> 
   <aspect name =“ WhereDoesTheTimeGo” /> 
</ aspects> 
</ aspectj>

我们可以指定aspectj 的配置文件，方式有两个

  1. 方式一是在目录下，新建META-INF目录，在该目录下aop-ajc.xml，aj打包时将自动选择xml文件作为aspectj的配置文件
  2. 方式二是，使用 -outxmlfile .xml 指定特定目录的xml文件作为aspectj的配置文件

我们简单修改ajc配置文件，增加weaver节点，以实现打印织入详情

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<aspectj> 
<aspects> 
   <aspect name =“ WhereDoesTheTimeGo” /> 
</ aspects> 
<weaver options =“-verbose  -showWeaveInfo” /> 
</aspectj>

这时候重新生成timing.jar ,再次执行，这时候输出的内容就多得多

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java -javaagent:<pathToAspectj>/lib/aspectjweaver.jar
     -classpath "code;timing.jar;<pathToAspectj>/lib/aspectjrt.jar" Simple
//输出
[AppClassLoader@9fbe93] info AspectJ Weaver Version DEVELOPMENT 
                        built on Wednesday Feb 25, 2009 at 21:17:03 GMT
[AppClassLoader@9fbe93] info register classloader sun.misc.Launcher$AppClassLoader@9fbe93
[AppClassLoader@9fbe93] info using configuration 
                        file:/C:/blog/timing.jar!/META-INF/aop-ajc.xml
[AppClassLoader@9fbe93] info register aspect WhereDoesTheTimeGo
[AppClassLoader@9fbe93] weaveinfo Join point 
                        'method-execution(void Simple.main(java.lang.String[]))'
                        in Type 'Simple' (Simple.java:3) advised by around advice
                        from 'WhereDoesTheTimeGo' (WhereDoesTheTimeGo.java:7)
[AppClassLoader@9fbe93] weaveinfo Join point 
                        'method-execution(void Simple.countSlow(int))' 
                        in Type 'Simple' (Simple.java:8) advised by around advice
                        from 'WhereDoesTheTimeGo' (WhereDoesTheTimeGo.java:7)
[AppClassLoader@9fbe93] weaveinfo Join point 
                        'method-execution(void Simple.countFast(int))' 
                        in Type 'Simple' (Simple.java:12) advised by around advice
                        from 'WhereDoesTheTimeGo' (WhereDoesTheTimeGo.java:7)
[AppClassLoader@9fbe93] weaveinfo Join point 
                        'method-execution(void Simple.count(int, int))' 
                        in Type 'Simple' (Simple.java:16) advised by around advice 
                        from 'WhereDoesTheTimeGo' (WhereDoesTheTimeGo.java:7)
[AppClassLoader@9fbe93] info processing reweavable type WhereDoesTheTimeGo: 
                        WhereDoesTheTimeGo.java
    execution(void Simple.count(int, int)) took 1ms
  execution(void Simple.countFast(int)) took 3ms
    execution(void Simple.count(int, int)) took 5005ms
  execution(void Simple.countSlow(int)) took 5005ms
execution(void Simple.main(String[])) took 5016ms

也可以开启 -debug 模式

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<aspectj>
 <aspects>
   <aspect name="WhereDoesTheTimeGo"/>
 </aspects>
 <weaver options="-debug">
   <exclude within="Simple">
 </weaver>
</aspectj>

通过这3个选项，我可以研究在尝试进行加载时编织时看到的任何基本问题。甚至更高级的选项可能涉及打开weaver跟踪或转储字节码（在编织之前和之后），但是当AspectJ编译器开发人员要求进行诊断时，通常会使用这些选项。

 
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