How to make the jvm load my java.lang.String instead of the one in rt.jar

问题

I studied java classloader rencently. Now I want to write a class that has the same package name and class name as one of class in rt.jar. For example, write a java.lang.String class by myself, and how to break the parents delegation model to make the jvm load my java.lang.String instead of the one in rt.jar.

Re-edit
Thx, tried. And ↓↓↓


    D:\>java -verbose -Xbootclasspath/p:D:/myrt.jar -jar exe.jar
    [Opened D:\myrt.jar]
    [Opened C:\java\jre\lib\rt.jar]
    [Loaded java.lang.Object from C:\java\jre\lib\rt.jar]
    [Loaded java.lang.String from D:\myrt.jar]
    [Loaded java.io.Serializable from C:\java\jre\lib\rt.jar]
    [Loaded java.lang.reflect.GenericDeclaration from C:\java\jre\lib\rt.jar]
    [Loaded java.lang.reflect.Type from C:\java\jre\lib\rt.jar]
    [Loaded java.lang.reflect.AnnotatedElement from C:\java\jre\lib\rt.jar]
    [Loaded java.lang.Class from C:\java\jre\lib\rt.jar]
    Invalid layout of java.lang.String at value
    #
    # A fatal error has been detected by the Java Runtime Environment:
    #
    #  Internal Error (javaClasses.cpp:136), pid=6968, tid=4116
    #  fatal error: Invalid layout of preloaded class
    #
    # JRE version:  (7.0_45-b18) (build )
    # Java VM: Java HotSpot(TM) 64-Bit Server VM (24.45-b08 mixed mode windows-amd64
     compressed oops)
    # Failed to write core dump. Minidumps are not enabled by default on client vers
    ions of Windows
    #
    # An error report file with more information is saved as:
    # D:\\hs_err_pid6968.log
    #
    # If you would like to submit a bug report, please visit:

回答1:

You can do this using the -Xbootclasspath/p option at JVM startup:

-Xbootclasspath/p:/path/to/yourimpl.jar

/p stands for "prepend".

Note: -Xbootclasspath isn't a standard java option, so JVMs by different vendors may not support it.

回答2:

Have a look at the java.lang.instrumentation API: you can implement a ClassFileTransformer which ignores the passed-in byte array and returns a bute array containing your own implementation of java.lang.String instead.

Better make sure that it's completely binary compatible with the original though or you will not get very far.

来源：https://stackoverflow.com/questions/22060752/how-to-make-the-jvm-load-my-java-lang-string-instead-of-the-one-in-rt-jar

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java

jvm

classloader