Spring之LoadTimeWeaver——一个需求引发的思考

public class DemoBean {

public void run() {

System.out.println("Run");

}

}

接着，我们编写分析方法执行效率的切面。

<span style="font-family: 'Courier New'; color: #646464; font-size: x-small;">@Aspect

StopWatch sw = new StopWatch(getClass().getSimpleName());

try {

sw.start(pjp.getSignature().getName());

return pjp.proceed();

} finally {

sw.stop();

System.out.println(sw.prettyPrint());

}

}

@Pointcut("execution(public com.shansun.. (..))")

public void profileMethod() {}

}</span>

前文提到，我们还需要一个aop.xml。这个文件要求放在META-INF/aop.xml路径下，以告知AspectJ Weaver我们需要把ProfilingAspect织入到应用的哪些类中。

<span style="font-family: 'Courier New'; color: #008080; font-size: x-small;"><!DOCTYPE aspectj PUBLIC "-//AspectJ//DTD//EN" " http://www.eclipse.org/aspectj/dtd/aspectj.dtd"> ;

<aspectj>

<weaver>

<include within="com.shansun..*" />

</weaver>

<aspects>

<!-- weave in just this aspect -->

<aspect name="com.shansun.multidemo.spring.ltw.ProfilingAspect" />

</aspects>

</aspectj></span>

目前为止，本次切面的“攻”和“受”都准备好了，我们还需要一个中间媒介——LoadTimeWeaver。我们将Spring的配置文件添加红色标识内容。

<?xml version="1.0" encoding="GBK"?>

<beans xmlns=" http://www.springframework.org/schema/beans "

xmlns:xsi=" http://www.w3.org/2001/XMLSchema-instance " xmlns:aop=" http://www.springframework.org/schema/aop "

xmlns:context="Index of /schema/context" xmlns:tx="Index of /schema/tx"

xsi:schemaLocation="

Index of /schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd

Index of /schema/aop http://www.springframework.org/schema/aop/spring-aop-2.5.xsd

Index of /schema/context http://www.springframework.org/schema/context/spring-context-2.5.xsd

http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.5.xsd"> ;

<context:load-time-weaver aspectj-weaving="autodetect" />

<context:component-scan base-package="com.shansun.multidemo"></context:component-scan>

</beans>

通过<context:load-time-weaveraspectj-weaving="on" />使spring开启loadtimeweaver,注意aspectj-weaving有三个选项: on, off, auto-detect, 如果设置为auto-detect, spring将会在classpath中查找aspejct需要的META-INF/aop.xml，如果找到则开启aspectj weaving,这个逻辑在LoadTimeWeaverBeanDefinitionParser#isAspectJWeavingEnabled方法中：

<span style="font-family: 'Courier New'; color: #7f0055; font-size: x-small;"><strong>protected boolean isAspectJWeavingEnabled(String value, ParserContext parserContext) {

if ("on".equals(value)) {

return true;

}

else if ("off".equals(value)) {

return false;

}

else {

// Determine default...

ClassLoader cl = parserContext.getReaderContext().getResourceLoader().getClassLoader();

return (cl.getResource(ASPECTJ_AOP_XML_RESOURCE) != null);

}

}</strong></span>

一切都准备就绪——切面类、aop.xml、Spring的配置，我们就创建一个main方法来掩饰LTW的功效吧。

public class Main {

public static void main(String[] args) {

ApplicationContext ctx = new ClassPathXmlApplicationContext("applicationContext.xml");

// DemoBean bean = (DemoBean) ctx.getBean("demoBean");

DemoBean bean = new DemoBean();

bean.run();

}

}

因为这个LTW使用成熟的AspectJ，我们并不局限于通知Spring beans的方法。所以上述代码中从ApplicationContext中获取Bean和直接实例化一个Bean的效果是一样的。

<span style="font-family: 'Courier New'; color: #ff0000; font-size: x-small;">Exception in thread "main" java.lang.IllegalStateException: Must start with Java agent to use InstrumentationLoadTimeWeaver. See Spring documentation.

at org.springframework.instrument.classloading.InstrumentationLoadTimeWeaver.addTransformer(InstrumentationLoadTimeWeaver.java:88)

at org.springframework.context.weaving.AspectJWeavingEnabler.postProcessBeanFactory(AspectJWeavingEnabler.java:69)

at org.springframework.context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors(AbstractApplicationContext.java:553)

at org.springframework.context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors(AbstractApplicationContext.java:536)

at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:362)

at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)

at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)

at com.shansun.multidemo.spring.Main.main(Main.java:25)</span>

必需使用java agent么？仔细观察异常的出处InstrumentationLoadTimeWeaver。再深入这个类的内容，发现异常是由下述方法抛出的。

public void addTransformer(ClassFileTransformer transformer) {

Assert.notNull(transformer, "Transformer must not be null");

FilteringClassFileTransformer actualTransformer =

new FilteringClassFileTransformer(transformer, this.classLoader);

synchronized (this.transformers) {

if (this.instrumentation == null) {

throw new IllegalStateException(

"Must start with Java agent to use InstrumentationLoadTimeWeaver. See Spring documentation.");

}

this.instrumentation.addTransformer(actualTransformer);

this.transformers.add(actualTransformer);

}

}

不难发现它在校验instrumentation是否为空的时候抛出的异常。有办法啦，重写InstrumentationLoadTimeWeaver的addTransformer方法，隐匿异常即可。

public class ExtInstrumentationLoadTimeWeaver extends

InstrumentationLoadTimeWeaver {

@Override

public void addTransformer(ClassFileTransformer transformer) {

try {

super.addTransformer(transformer);

} catch (Exception e) {}

}

}

这时，我们还需要做一件事，将Spring配置文件中的load-time-weaver入口设置为我们刚自定义的ExtInstrumentationLoadTimeWeaver即可。

<?xml version="1.0" encoding="GBK"?>

<beans xmlns=" http://www.springframework.org/schema/beans "

xmlns:xsi=" http://www.w3.org/2001/XMLSchema-instance " xmlns:aop="Index of /schema/aop"

xmlns:context="Index of /schema/context" xmlns:tx="Index of /schema/tx"

xsi:schemaLocation="

Index of /schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd

Index of /schema/aop http://www.springframework.org/schema/aop/spring-aop-2.5.xsd

Index of /schema/context http://www.springframework.org/schema/context/spring-context-2.5.xsd

Index of /schema/tx http://www.springframework.org/schema/tx/spring-tx-2.5.xsd"> ;

<context:load-time-weaver weaver-class="com.shansun.multidemo.spring.ExtInstrumentationLoadTimeWeaver" aspectj-weaving="autodetect" />

<context:component-scan base-package="com.shansun.multidemo"></context:component-scan>

</beans>

再次运行我们的main方法，发现只输出了如下结果，切面没有起作用。