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Aside from recompiling rt.jar is there any way I can replace the currentTimeMillis() call with one of my own?

1# The right way to do it is use a Clock object and abstract time.

I know it but we'll be running code developed by an endless number of developers that have not implemented Clock or have made an implementation of their own.

2# Use a mock tool like JMockit to mock that class.

Even though that only works with Hotspot disabled -Xint and we have success using the code bellow it does not "persist" on external libraries. Meaning that you'd have to Mock it everywhere which, as the code is out of our control, is not feasible. All code under main() does return 0 milis (as from the example) but a new DateTime() will return the actual system millis.

 @MockClass(realClass = System.class) public class SystemMock extends MockUp<System> { // returns 1970-01-01 @Mock public static long currentTimeMillis() { return 0; } }

3# Re-declare System on start up by using -Xbootclasspath/p (edited)

While possible, and though you can create/alter methods, the one in question is declared as public static native long currentTimeMillis();. You cannot change it's declaration without digging into Sun's proprietary and native code which would make this an exercise of reverse engineering and hardly a stable approach. All recent SUN JVM crash with the following error:

 EXCEPTION_ACCESS_VIOLATION (0xc0000005) at pc=0x00000, pid=4668, tid=5736

4# Use a custom ClassLoader (new test as suggested on the comments)

While trivial to replace the system CL using -Djava.system.class.loader JVM actually loads up the custom classLoader resorting to the default classLoader and System is not even pushed trough the custom CL.

 public class SimpleClassLoader extends ClassLoader { public SimpleClassLoader(ClassLoader classLoader) { super(classLoader); } @Override public Class<?> loadClass(String name) throws ClassNotFoundException { return super.loadClass(name); } }

We can see that java.lang.System is loaded from rt.jar using java -verbose:class 

Line 15: [Loaded java.lang.System from C:\jdk1.7.0_25\jre\lib\rt.jar]

I'm running out of options.
Is there some approach I'm missing?

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  • AspectJ might be an option. Commented Aug 14, 2013 at 18:50
  • Out of flags, but possible duplicate of stackoverflow.com/questions/2001671/… . Commented Aug 14, 2013 at 18:51
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    Well, I suppose you could always use CGLIB and just return your own value using their method interceptor. Commented Aug 14, 2013 at 18:55
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    well, replacing the system classloader using -Djava.system.class.loader option? Commented Aug 14, 2013 at 19:20
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    Also as for the ClassLoader approach, you can specify a custom system class loader on the JVM command line; that should affect all libraries loaded by that JVM: java -Djava.system.class.loader=your.package.CustomClassLoader ... Commented Aug 14, 2013 at 19:20

4 Answers 4

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You could use an AspectJ compiler/weaver to compile/weave the problematic user code, replacing the calls to java.lang.System.currentTimeMillis() with your own code. The following aspect will just do that:

public aspect CurrentTimeInMillisMethodCallChanger { long around(): call(public static native long java.lang.System.currentTimeMillis()) && within(user.code.base.pckg.*) { return 0; //provide your own implementation returning a long } }
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While this looks very promising it still fails on the scenario where the user captures date from an external library. Say Joda was used. new DateTime() would give back the correct system time, right? I'm asking because I've never used AspectJ before and while I was able to run and compile the code you provided (thanks, by the way) I was unable to have Joda report milis as 0. Joda is not compiled/weaved by AspectJ and, therefore, feeds back the system time. Am I right in assuming this?
You can also weave the Joda library. The org.joda.time.DateTimeUtils.SystemMillisProvider is using System.currentTimeMillis(), so if it's replaced (either by classpath order, classloading, or weaving), it will do whatever you want. There are also other sublcasses of org.joda.time.DateTimeUtils.MillisProvider in Joda, so I suppose it can be configured to use another provider an then you don't even have to change the SystemMillisProvider. If other libraries are involved, they can also be changed by weaving, though you'll end up with multiple special weaved version of those libraries.
Every single library out there, even Date(), fall back to System.currentTimeMilis() so if we can change it they should all work as expected. To run your example I read the docs and compiled the code using ajc. From this reply I inferred that you can "weave" at runtime. I will be reading the docs to try and do it, if you can expand the answer or give me some points on the right track I'd be much appreciated. Thanks!
You can use load time weaving, but it might be a bit more difficult to set up. Though I never tried it, I would advise you strongly against trying to weave JDK classes at load time, because the LTW agent itself is loaded by the JDK at bootstrap, so you would end up with some JDK classes weaved and some not. I see a lot more potential problems with that route. The route I would choose is to weave at compile/build time, and weave all points of code at the border between user code and JDK code, so that all invocations that end up calling System.currentTimeMillis() get replaced by your custom code
I've tried both but went with LTW (load-time-weaving) as it provides exceptional adaptability to all sorts of possible user-loaded classes (including those loaded with custom class-loaders). Re-defined currentTimeMillis, Date and Calendar. Joda and all similar work instantly out-of-the-box as LTW weaves their calls. Was also able to capture wait and sleep that will have to be re-defined to make things fully functional (as Timers rely on wait) but, so far, this route has produced wonderful results. Thank you very much for the help and for providing relevant and extremely helpful code.
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I'm not 100% sure if I oversee something here, but you can create your own System class like this:

public static class System { static PrintStream err = System.err; static InputStream in = System.in; static PrintStream out = System.out; static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length) { System.arraycopy(src, srcPos, dest, destPos, length); } // ... and so on with all methods (currently 26) except `currentTimeMillis()` static long currentTimeMillis() { return 4711L; // Your application specific clock value } }

than import your own System class in every java file. Reorganize imports in Eclipse should do the trick. And than all java files should use your applicatikon specific System class.

As I said, not a nice solution because you will need to maintain your System class whenever Java changes the original one. Also you must make sure, that always your class is used.

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Thanks for the input but this fails on number #1. We can't replace the code because it's uploaded by an endless number of different developers. And, if we could, we should use an abstraction of date, like Clock and have it return a true or fake time depending on our needs, that would be less messy than re-defining System. I'm still slowly looking into the other feedback and will report any findings.
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As discussed in the comments, it is possible that option #3 in the original question has actually worked, successfully replacing the default System class.

If that is true, then application code which calls currentTimeMillis() will be calling the replacement, as expected.

Perhaps unexpectedly, core classes like java.util.Timer would also get the replacement!

If all of the above are true, then the root cause of the crash could be the successful replacement of the System class.

To test, you could instead replace System with a copy that is functionally identical to the original to see if the crashes disappear.

Unfortunately, if this answer turns out to be correct, it would seem that we have a new question. :) It might go like this:

"How do you provide an altered System.currentTimeMillis() to application classes, but leave the default implementation in place for core classes?"

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You were right in assuming I had been able to replace System but had an error. If I copy Sun's System.java code I can mess with it, create my own methods, change implemented methods but, unfortunately, currentTimeMillis() is declared as a public static native long and I can't mess with the declaration as the VM crashes. I've dug into it a bit but eventually I'd have to change native proprietary code which, while may be feasible, makes it a long shot. Will try other approaches and keep reporting. Will also edit question accordingly. Thanks.
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i've tried using javassist to remove the native currentTimeMills, add a pure java one and load it using bootclasspath/p, but i got the same exception access violation as you did. i believe that's probably because of the native method registerNatives that's called in the static block but it's really too much to disassemble the native library.

so, instead of changing the System.currentTimeMills, how about changing the user code? if the user code already compiled (you don't have source code), we can use tools like findbugs to identify the use of currentTimeMillis and reject the code (maybe we can even replace the call to currentTimeMills with your own implementation).

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As stated on #1 there is no way we can change the user-code. And while I can relate to your logic - decompile the whole thing - change all calls to currentTimeMilis()- compile again; it looks way to messy. ASM and BCEL may be better alternatives though I'm still investigating. Many thanks for the input.
FWIW i managed to do this with Javassist in a Java agent premain using instrumentation.retransformClasses(java.lang.System.class); with much less trouble than i had expected.

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