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But That's Impossible, or Finding Out That the JIT has Broken Your Code

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But That's Impossible, or Finding Out That the JIT has Broken Your Code

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Every now and then you look at some code and think that it cannot be possibly be wrong. Once you have ruled out a simple programmer screw up / enemy action in code (Make sure you read Java Puzzlers or similar) or a concurrency issue (Read Java Concurrency or go on Dr Heniz excellent course) you should sit back and take a few days and then starting thinking about whether the JDK is indeed out to get you. I haven't seen one in the wild in my 18 odd years as a Java programmer so it kinda took me by surprise.

If you are running against JDK 8 in a large scale Swing application you might eventually see the following exception, lots of lots of times. (Unless you forgot the lesson learned in my previous blog in your logging code, in which case you might see lots of ArrayOfOutBoundsException)

Caused by: java.lang.NullPointerException 
    at javax.swing.text.GlyphView.getBreakSpot(GlyphView.java:799) 
    at javax.swing.text.GlyphView.getBreakWeight(GlyphView.java:724) 
    at javax.swing.text.FlowView$LogicalView.getPreferredSpan(FlowView.java:733) 
    at javax.swing.text.FlowView.calculateMinorAxisRequirements(FlowView.java:233) 
    at javax.swing.text.ParagraphView.calculateMinorAxisRequirements(ParagraphView.java:717) 
    at javax.swing.text.BoxView.checkRequests(BoxView.java:935) 
    at javax.swing.text.BoxView.getMinimumSpan(BoxView.java:568) 
    at javax.swing.text.BoxView.calculateMinorAxisRequirements(BoxView.java:903) 
    at javax.swing.text.BoxView.checkRequests(BoxView.java:935) 
    at javax.swing.text.BoxView.setSpanOnAxis(BoxView.java:343) 
    at javax.swing.text.BoxView.layout(BoxView.java:708) 
    at javax.swing.text.BoxView.setSize(BoxView.java:397) 
    ...

This error is particularly insidious because it takes around ten minutes to show itself and sometimes not at all. If you look at the code for this class the line in question, starts with "startsFrom = break", only accesses two local variables both of which have been previously referenced in the method.

            Segment s = getText(pstart, pend); 
            s.first(); 
            BreakIterator breaker = getBreaker(); 
            breaker.setText(s); 

            // Backward search should start from end+1 unless there's NO end+1 
            int startFrom = end + (pend > end ? 1 : 0); 
            for (;;) { 
                startFrom = breaker.preceding(s.offset + (startFrom - pstart))   
                          + (pstart - s.offset); 
                if (startFrom > start) { 
                    // The break spot is within the view 
                    bs[ix++] = startFrom; 
                } else { 
                    break; 
                } 
            } 

The most direct way to rule out a JIT error is to disable compilation for just this single method, here is an example; but you can fine more in the documentation for the command line java tool.

javaThing -XX:CompileCommand=exclude,javax/swing/text/GlyphView,getBreakSpot  

When this parameter is added, the problem goes away. - since we have ruled out enemy action by code or a concurrency issue we can be more sure this is a JIT issue. Now as part of the bug logging for this I output diagnostics for this single method, and found out that the problem, didn't occur until the method was JITted for a fifth time.

javaThing -XX:CompileCommand=print,javax/swing/text/GlyphView,getBreakSpot  

Here is some diagnostic output seen with the above command:

Compiled method (c2)  914078 33142       4       javax.swing.text.GlyphView::getBreakSpot (247 bytes)
 total in heap  [0x00002aaab0749e10,0x00002aaab0750fe0] = 29136
 relocation     [0x00002aaab0749f38,0x00002aaab074a1e8] = 688
 constants      [0x00002aaab074a200,0x00002aaab074a2a0] = 160
 main code      [0x00002aaab074a2a0,0x00002aaab074cde0] = 11072
 stub code      [0x00002aaab074cde0,0x00002aaab074ce40] = 96
 oops           [0x00002aaab074ce40,0x00002aaab074ce58] = 24
 metadata       [0x00002aaab074ce58,0x00002aaab074d058] = 512
 scopes data    [0x00002aaab074d058,0x00002aaab074ea20] = 6600
 scopes pcs     [0x00002aaab074ea20,0x00002aaab0750c50] = 8752
 dependencies   [0x00002aaab0750c50,0x00002aaab0750c80] = 48
 handler table  [0x00002aaab0750c80,0x00002aaab0750e90] = 528
 nul chk table  [0x00002aaab0750e90,0x00002aaab0750fe0] = 336
OopMapSet contains 113 OopMaps
#0 
OopMap{[8]=Oop [32]=Oop [40]=Oop off=892}
#1 
OopMap{[32]=Oop [40]=Oop off=960}
#2 
OopMap{[32]=Oop [40]=Oop off=980}
#3 
OopMap{[32]=Oop [40]=Oop [48]=Oop off=1048}
#4 
OopMap{[32]=Oop [40]=Oop [48]=Oop off=1084}
#5 
OopMap{[0]=Oop [24]=Oop [48]=Oop [56]=Oop [80]=Oop off=2500}
#6 
OopMap{rbx=Oop rdi=Oop [32]=Oop [40]=Oop [112]=Oop off=2533}
#7 
OopMap{rbx=Oop rdi=Oop r14=Oop [32]=Oop [112]=Oop off=3081}
#8 
OopMap{rbx=Oop rdi=Oop r14=Oop [32]=Oop [40]=Oop [112]=Oop off=3190}
#9 
OopMap{[8]=Oop [32]=Oop [40]=Oop off=4408}
#10 
OopMap{[32]=Oop [40]=Oop [48]=Oop off=4640}
#11 
OopMap{rbp=Oop [16]=Oop [24]=Oop [40]=Oop [64]=Oop off=5232}
#12 
OopMap{rbp=Oop [0]=NarrowOop [32]=Oop off=5364}
#13 
OopMap{[32]=Oop [40]=Oop [48]=Oop off=5408}
#14 
OopMap{rbp=Oop [32]=Oop [40]=Oop [48]=Oop off=5436}
#15 
OopMap{rbp=Oop [32]=Oop [40]=Oop [48]=Oop off=5468}
#16 
OopMap{rbp=Oop [32]=Oop [40]=Oop [48]=Oop off=5524}
#17 
OopMap{rbp=Oop [32]=Oop [40]=Oop [48]=Oop [88]=Oop off=5552}
#18 
OopMap{[32]=Oop [40]=Oop [48]=Oop [64]=Oop [72]=Derived_oop_[64] [112]=Oop off=5608}
#19 
OopMap{[8]=Oop [32]=Oop off=5680}
#20 
OopMap{rbp=Oop off=5720}
#21 
OopMap{rbp=Oop off=5752}
#22 
OopMap{rbp=Oop [24]=NarrowOop [28]=NarrowOop [32]=Oop [40]=Oop [48]=Oop [56]=Oop [64]=Oop [88]=Oop off=5812}
#23 
OopMap{rbp=Oop [16]=Oop [24]=Oop [40]=Oop [64]=Oop [88]=Oop off=5960}
#24 
OopMap{[0]=Oop [24]=Oop [48]=Oop [56]=Oop [72]=Oop [88]=NarrowOop off=6056}
#25 
OopMap{[40]=Oop off=6088}
#26 
OopMap{[0]=Oop off=6120}
#27 
OopMap{[8]=Oop [24]=Oop [56]=Oop [72]=Oop [112]=Oop off=6216}
#28 
OopMap{[0]=Oop [32]=NarrowOop [40]=Oop off=6284}
#29 
OopMap{rbp=Oop [16]=Oop [40]=Oop [64]=Oop [112]=Oop off=6384}
#30 
OopMap{[0]=Oop off=6412}
#31 
OopMap{[0]=Oop [16]=Oop [32]=NarrowOop [40]=Oop [48]=Oop off=6488}
#32 
OopMap{rbp=Oop [16]=Oop [40]=Oop [48]=Oop off=6560}
#33 
OopMap{[32]=Oop [40]=Oop [48]=Oop [64]=Oop [112]=Oop off=6608}
#34 
OopMap{[8]=Oop [28]=NarrowOop [32]=Oop [40]=Oop [48]=Oop off=6768}
#35 
OopMap{rbp=NarrowOop [0]=Oop [16]=Oop [32]=Oop [40]=NarrowOop off=6860}
#36 
OopMap{[0]=Oop [16]=Oop [32]=NarrowOop [40]=Oop [48]=Oop off=6988}
#37 
OopMap{rbp=Oop [32]=Oop off=7024}
#38 
OopMap{rbp=NarrowOop [0]=Oop [24]=Oop [32]=Oop off=7260}
#39 
OopMap{rbp=NarrowOop [0]=Oop [24]=Oop [32]=Oop off=7344}
#40 
OopMap{rbp=Oop [16]=Oop [24]=Oop [40]=Oop [60]=NarrowOop [64]=Oop off=7452}
#41 
OopMap{rbp=NarrowOop [32]=Oop off=7476}
#42 
OopMap{rbp=NarrowOop [0]=Oop off=7524}
#43 
OopMap{[32]=Oop [40]=Oop [48]=Oop off=7588}
#44 
OopMap{[32]=Oop [40]=Oop [48]=Oop off=7616}
#45 
OopMap{[32]=Oop [40]=Oop [48]=Oop off=7632}
#46 
OopMap{rbp=NarrowOop [32]=Oop off=7676}
#47 
OopMap{rbp=NarrowOop [0]=Oop off=7724}
#48 
OopMap{[0]=Oop [16]=Oop [28]=NarrowOop [40]=Oop [48]=Oop [56]=NarrowOop [64]=Oop off=7868}
#49 
OopMap{[8]=Oop [28]=NarrowOop [32]=Oop [40]=Oop [48]=Oop [56]=Oop off=7916}
#50 
OopMap{rbp=Oop [16]=Oop [24]=NarrowOop off=8016}
#51 
OopMap{rbp=Oop [16]=Oop [28]=NarrowOop off=8080}
#52 
OopMap{rbp=NarrowOop [0]=Oop [24]=Oop [32]=Oop off=8152}
#53 
OopMap{rbp=Oop [8]=NarrowOop off=8212}
#54 
OopMap{rbp=NarrowOop [32]=Oop off=8236}
#55 
OopMap{rbp=Oop [16]=NarrowOop off=8272}
#56 
OopMap{rbp=NarrowOop [0]=Oop off=8320}
#57 
OopMap{rbp=Oop [12]=NarrowOop off=8360}
#58 
OopMap{rbp=NarrowOop [32]=Oop off=8400}
#59 
OopMap{rbp=Oop [12]=NarrowOop off=8460}
#60 
OopMap{rbp=NarrowOop [0]=Oop off=8508}
#61 
OopMap{rbp=Oop [24]=NarrowOop [40]=Oop off=8572}
#62 
OopMap{rbp=Oop off=8600}
#63 
OopMap{rbp=Oop [8]=Oop [28]=NarrowOop off=8640}
#64 
OopMap{rbp=Oop [8]=Oop [20]=NarrowOop [112]=Oop off=8704}
#65 
OopMap{rbp=Oop [16]=Oop [24]=Oop [48]=Oop off=8788}
#66 
OopMap{rbp=Oop [16]=Oop [24]=Oop [40]=Oop [64]=Oop off=8912}
#67 
OopMap{rbp=Oop [16]=Oop [24]=Oop [40]=Oop [64]=Oop off=9036}
#68 
OopMap{rbp=Oop [16]=Oop [24]=Oop [40]=Oop [64]=Oop off=9160}
#69 
OopMap{rbp=Oop [16]=Oop [24]=Oop [40]=Oop [64]=Oop off=9284}
#70 
OopMap{rbp=Oop [16]=Oop [24]=Oop [40]=Oop [64]=Oop off=9408}
#71 
OopMap{rbp=Oop [16]=Oop [24]=Oop [40]=Oop [64]=Oop off=9532}
#72 
OopMap{off=9556}
#73 
OopMap{off=9580}
#74 
OopMap{off=9604}
#75 
OopMap{[112]=Oop off=9628}
#76 
OopMap{rbp=Oop [8]=Oop [24]=Oop [32]=NarrowOop off=9696}
#77 
OopMap{rbp=Oop [8]=Oop [24]=NarrowOop off=9760}
#78 
OopMap{off=9784}
#79 
OopMap{off=9812}
#80 
OopMap{off=9836}
#81 
OopMap{off=9860}
#82 
OopMap{off=9884}
#83 
OopMap{off=9908}
#84 
OopMap{off=9932}
#85 
OopMap{off=9956}
#86 
OopMap{off=9980}
#87 
OopMap{off=10004}
#88 
OopMap{off=10028}
#89 
OopMap{rbp=Oop [16]=Oop [28]=NarrowOop off=10092}
#90 
OopMap{rbp=Oop [16]=Oop [24]=Oop [48]=Oop off=10176}
#91 
OopMap{off=10200}
#92 
OopMap{off=10224}
#93 
OopMap{off=10248}
#94 
OopMap{off=10272}
#95 
OopMap{off=10296}
#96 
OopMap{off=10320}
#97 
OopMap{off=10344}
#98 
OopMap{off=10368}
#99 
OopMap{off=10392}
#100 
OopMap{off=10416}
#101 
OopMap{off=10440}
#102 
OopMap{off=10464}
#103 
OopMap{off=10488}
#104 
OopMap{off=10512}
#105 
OopMap{off=10536}
#106 
OopMap{off=10560}
#107 
OopMap{off=10584}
#108 
OopMap{off=10608}
#109 
OopMap{off=10632}
#110 
OopMap{off=10656}
#111 
OopMap{off=10680}
#112 
OopMap{off=11028}
java.lang.NullPointerException
 at javax.swing.text.GlyphView.getBreakSpot(GlyphView.java:799)
 at javax.swing.text.GlyphView.getBreakWeight(GlyphView.java:724)
 at javax.swing.text.html.InlineView.getBreakWeight(InlineView.java:150)
 at javax.swing.text.FlowView$LogicalView.getPreferredSpan(FlowView.java:733)
 at javax.swing.text.FlowView.calculateMinorAxisRequirements(FlowView.java:233)
 at javax.swing.text.ParagraphView.calculateMinorAxisRequirements(ParagraphView.java:717)
 at javax.swing.text.html.ParagraphView.calculateMinorAxisRequirements(ParagraphView.java:157)
 at javax.swing.text.BoxView.checkRequests(BoxView.java:935)
 at javax.swing.text.BoxView.getMinimumSpan(BoxView.java:568)
 at javax.swing.text.html.ParagraphView.getMinimumSpan(ParagraphView.java:270)
 at javax.swing.text.BoxView.calculateMinorAxisRequirements(BoxView.java:903)
Now I am still working with the JDK team for a fix for this one; but I do feel I have discovered a useful set of tools for providing some evidence that the JIT compiler is causing my bad day. And more importantly I have a workaround so I can run my tests until this is resolved.


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Published at DZone with permission of Gerard Davison, DZone MVB. See the original article here.

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