Byteman - Carving up your Java code

Brown Bag talk I gave on first steps with Byteman.

Transcript

1. Byteman - Carving up your Java code
   Chris Sinjakli
   

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2. A tracing/debugging tool
   Runtime manipulation of Java code
   

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3. Java Agent API
   Muck around with the bytecode to
   your heart’s content
   

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4. Java Agent API
   package java.lang.instrument;
   interface ClassFileTransformer
   {
   byte[] transform(ClassLoader loader, String
   className, Class> classBeingRedefined,
   ProtectionDomain protectionDomain,
   byte[] classfileBuffer);
   }
   

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5. Who does this?
   • About a million tutorials
   – Usually print the class name
   – At best adding simple trace code to methods
   • Newer, cooler mocking frameworks
   – Powermock, Mockito, JMockit
   – Better approach then JMock & friends (which use
   Proxy + CGLIB)
   – Mock final classes/methods, plus other stuff
   • Pure Java profilers?
   

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6. Not that convenient
   Who wants to spend their time
   dealing with bytecode?
   

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7. Byteman
   Muck around with the Java code to
   your heart’s content
   

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8. How?
   • Specify byteman.jar as a Java Agent when you
   start your app
   • Provide a Byteman rule script
   • Can provide these scripts later (to a running
   app) – app must be started with Byteman
   listener
   

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9. Simple program
   package org.my
   class AppMain
   {
   public static void main(String[] args)
   {
   for (int i = 0; i < args.length; i++) {
   System.out.println(args[i]);
   }
   }
   }
   

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10. Run it
    > java org.my.AppMain foo bar baz
    foo
    bar
    baz
    >
    

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11. Byteman rule script
    RULE trace main entry
    CLASS AppMain
    METHOD main
    AT ENTRY
    IF true
    DO traceln("entering main")
    ENDRULE
    

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12. Byteman rule script
    RULE trace main exit
    CLASS AppMain
    METHOD main
    AT EXIT
    IF true
    DO traceln("exiting main")
    ENDRULE
    

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13. Run it
    > java -
    javaagent:byteman.jar=script:appmain.btm org.m
    y.AppMain foo bar baz
    entering main
    foo
    bar
    baz
    exiting main
    >
    

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14. Worth mentioning
    • You can use it on classes you don’t have the
    source for
    • You can even use this on core Java classes
    (java.lang) – Byteman makes you confirm
    • Works on interfaces too (all implementing classes
    get your injected code)
    • Much more sophisticated rule scripts
    – Different “AT” points in method
    – “IF” statements that do something
    – Access to local variables (eg “AFTER WRITE $foo”)
    

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15. Testing
    

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16. Testing support
    • Comes with a JUnit Runner
    • Define the rules (from earlier scripts) as test
    annotations
    • Useful for fault injection (integration tests
    where you’re not mocking everything out?)
    

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17. Testing support
    @Test
    @BMRule(name=“foo”,
    targetClass = “FileInputStream”,
    targetMethod=“(File)”,
    condition=“$1.getName().equals(\“doesnt_exist.txt\”)”,
    action=“throw new FileNotFoundException(\“It’s not there!\”)”)
    public void testReadFile() {
    ...
    myClass.processFile(“doesnt_exist.txt”); // Throws an exception
    ...
    }
    

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18. Testing support
    • Rule in previous example will only be in place
    for that test
    – Can annotate class instead to apply to all tests
    • There’s also @BMScript to import external
    rule scripts
    • Relatively simple to run with maven (config
    samples on website)
    

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19. Summary
    • Seems cool
    • Not sure what I’d actually use it for
    • Integration tests most likely candidate
    

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20. Questions?
    

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