在字節(jié)碼這個(gè)層面對(duì)類和方法進(jìn)行修改的技術(shù)，能夠在不影響編譯的情況下，修改字節(jié)碼。可以理解spring的aop技術(shù)
如何實(shí)現(xiàn)
1.實(shí)現(xiàn)javaagent需要實(shí)現(xiàn)premain方法
2.必須在manifest.mf文件中有premain-class
demo實(shí)現(xiàn)
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agent package com.xueyou.demo.agent; import javassist.classpool; import javassist.ctclass; import javassist.ctmethod; import javassist.ctnewmethod; import java.lang.instrument.classfiletransformer; import java.lang.instrument.illegalclassformatexception; import java.security.protectiondomain; public class firstagent implements classfiletransformer { public final string injectedclassname = "com.xueyou.agentdemo.app"; public final string methodname = "hello"; public byte[] transform(classloader loader, string classname, class<?> classbeingredefined, protectiondomain protectiondomain, byte[] classfilebuffer) throws illegalclassformatexception { classname = classname.replace("/", "."); // system.out.println(classname); if (classname.equals(injectedclassname)) { ctclass ctclass = null; try { ctclass = classpool.getdefault().get(classname);// 使用全稱,用于取得字節(jié)碼類<使用javassist> ctmethod ctmethod = ctclass.getdeclaredmethod(methodname);// 得到這方法實(shí)例 ctmethod.insertbefore("system.out.println(11111111);"); return ctclass.tobytecode(); } catch (exception e) { system.out.println(e.getmessage()); e.printstacktrace(); } } return null; } } |
預(yù)加載類
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package com.xueyou.demo;import com.xueyou.demo.agent.firstagent;import java.lang.instrument.instrumentation;/** * hello world! */public class app { public static void premain(string agentops, instrumentation inst) { system.out.println("=========premain方法執(zhí)行========"); system.out.println(agentops); // 添加transformer inst.addtransformer(new firstagent()); }} |
maven指定預(yù)加載類
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<build> <plugins> <plugin> <groupid>org.apache.maven.plugins</groupid> <artifactid>maven-compiler-plugin</artifactid> <configuration> <source>1.8</source> <target>1.8</target> <encoding>utf-8</encoding> </configuration> </plugin> <plugin> <groupid>org.apache.maven.plugins</groupid> <artifactid>maven-shade-plugin</artifactid> <version>3.0.0</version> <executions> <execution> <phase>package</phase> <goals> <goal>shade</goal> </goals> <configuration> <transformers> <transformer implementation="org.apache.maven.plugins.shade.resource.manifestresourcetransformer"> <manifestentries> <premainclass>com.xueyou.demo.app</premain-class> </manifestentries> </transformer> </transformers> </configuration> </execution> </executions> </plugin> </plugins> </build> |
如何使用
這里有兩種方式指定javaagent：
1）使用命令行：java -javaagent:xxx.jar ddd.jar
2）使用idea的vm option選項(xiàng)。
總結(jié)
以上所述是小編給大家介紹的java agent 使用及實(shí)現(xiàn)代碼，希望對(duì)大家有所幫助，如果大家有任何疑問請(qǐng)給我留言，小編會(huì)及時(shí)回復(fù)大家的。在此也非常感謝大家對(duì)服務(wù)器之家網(wǎng)站的支持！
原文鏈接：https://segmentfault.com/a/1190000015765838
