ESC/Java extended static checking for Java Erik Poll, Joe Kiniry, David Cok University of Nijmegen; Eastman Kodak Company

Transcription

1 ESC/Java extended static checking for Java Erik Poll, Joe Kiniry, David Cok University of Nijmegen; Eastman Kodak Company Erik Poll - JML p.1/??

2 ESC/Java Extended static checker by Rustan Leino et.al. [Compaq]. tries to prove correctness of specifications, at compile-time, fully automatically Erik Poll - JML p.2/??

3 ESC/Java Extended static checker by Rustan Leino et.al. [Compaq]. tries to prove correctness of specifications, at compile-time, fully automatically not sound, not complete, but finds lots of potential bugs quickly Erik Poll - JML p.2/??

4 ESC/Java Extended static checker by Rustan Leino et.al. [Compaq]. tries to prove correctness of specifications, at compile-time, fully automatically not sound, not complete, but finds lots of potential bugs quickly good at proving absence of runtime exceptions (eg Null-, ArrayIndexOutOfBounds-, ClassCast-) and verifying relatively simple properties. Erik Poll - JML p.2/??

5 ESC/Java Extended static checker by Rustan Leino et.al. [Compaq]. tries to prove correctness of specifications, at compile-time, fully automatically not sound, not complete, but finds lots of potential bugs quickly good at proving absence of runtime exceptions (eg Null-, ArrayIndexOutOfBounds-, ClassCast-) and verifying relatively simple properties. ESC/Java only supported a subset of full JML, but ESC/Java2 by Joe Kiniry [KUN] & David Cok [Kodak] remedies this. Erik Poll - JML p.2/??

6 Important differences: static checking vs runtime checking ESC/Java checks specs at compile-time, jmlc checks specs at run-time ESC/Java proves correctness of specs, jml only tests correctness of specs. Hence ESC/Java independent of any test suite, results of runtime testing only as good as the test suite, ESC/Java provided higher degree of confidence. Erik Poll - JML p.3/??

7 ESC/Java demo class Bag { int[] a; int n; int extractmin() { int m = Integer.MAX_VALUE; int mindex = 0; for (int i = 1; i <= n; i++) { if (a[i] < m) { mindex =i; m = a[i]; } } n--; a[mindex] = a[n]; return m; } Erik Poll - JML p.4/??

8 ESC/Java demo class Bag { int[] a; int n; int extractmin() { int m = Integer.MAX_VALUE; int mindex = 0; for (int i = 1; i <= n; i++) { if (a[i] < m) { mindex =i; m = a[i]; } } n--; a[mindex] = a[n]; return m; } Warning: possible null deference. Plus other warnings Erik Poll - JML p.5/??

9 ESC/Java demo class Bag { int[] a; //@ invariant a!= null; int n; int extractmin() { int m = Integer.MAX_VALUE; int mindex = 0; for (int i = 1; i <= n; i++) { if (a[i] < m) { mindex =i; m = a[i]; } } n--; a[mindex] = a[n]; return m; } Erik Poll - JML p.6/??

10 ESC/Java demo class Bag { int[] a; //@ invariant a!= null; int n; int extractmin() { int m = Integer.MAX_VALUE; int mindex = 0; for (int i = 1; i <= n; i++) { if (a[i] < m) { mindex =i; m = a[i]; } } n--; a[mindex] = a[n]; return m; } Warning: Array index possibly too large Erik Poll - JML p.7/??

11 ESC/Java demo class Bag { int[] a; //@ invariant a!= null; int n; //@ invariant 0 <= n && n <= a.length; int extractmin() { int m = Integer.MAX_VALUE; int mindex = 0; for (int i = 1; i <= n; i++) { if (a[i] < m) { mindex =i; m = a[i]; } } n--; a[mindex] = a[n]; return m; } Erik Poll - JML p.8/??

12 ESC/Java demo class Bag { int[] a; //@ invariant a!= null; int n; //@ invariant 0 <= n && n <= a.length; int extractmin() { int m = Integer.MAX_VALUE; int mindex = 0; for (int i = 1; i <= n; i++) { if (a[i] < m) { mindex =i; m = a[i]; } } n--; a[mindex] = a[n]; return m; } Warning: Array index possibly too large Erik Poll - JML p.9/??

13 ESC/Java demo class Bag { int[] a; //@ invariant a!= null; int n; //@ invariant 0 <= n && n <= a.length; int extractmin() { int m = Integer.MAX_VALUE; int mindex = 0; for (int i = 0; i < n; i++) { if (a[i] < m) { mindex =i; m = a[i]; } } n--; a[mindex] = a[n]; return m; } Erik Poll - JML p.10/??

14 ESC/Java demo class Bag { int[] a; //@ invariant a!= null; int n; //@ invariant 0 <= n && n <= a.length; int extractmin() { int m = Integer.MAX_VALUE; int mindex = 0; for (int i = 0; i < n; i++) { if (a[i] < m) { mindex =i; m = a[i]; } } n--; a[mindex] = a[n]; return m; } Warning: Possible negative array index Erik Poll - JML p.11/??

15 class Bag { int[] a; //@ invariant a!= null; ESC/Java demo int n; //@ invariant 0 <= n && n <= a.length; //@ requires n > 0; int extractmin() { int m = Integer.MAX_VALUE; int mindex = 0; for (int i = 0; i < n; i++) { if (a[i] < m) { mindex =i; m = a[i]; } } n--; a[mindex] = a[n]; return m; } Erik Poll - JML p.12/??

16 class Bag { int[] a; //@ invariant a!= null; ESC/Java demo int n; //@ invariant 0 <= n && n <= a.length; //@ requires n > 0; int extractmin() { int m = Integer.MAX_VALUE; int mindex = 0; for (int i = 0; i < n; i++) { n--; if (a[i] < m) { mindex =i; m = a[i]; } } a[mindex] = a[n]; return m; } No more warnings about this code Erik Poll - JML p.13/??

17 class Bag { int[] a; //@ invariant a!= null; ESC/Java demo int n; //@ invariant 0 <= n && n <= a.length; //@ requires n > 0; int extractmin() { int m = Integer.MAX_VALUE; int mindex = 0; for (int i = 0; i < n; i++) { if (a[i] < m) { mindex =i; m = a[i]; } } n--; a[mindex] = a[n]; return m; }... but warnings about calls to extractmin() that do not ensure precondition Erik Poll - JML p.14/??

18 Some points to note ESC/Java forces one to specify some properties. Erik Poll - JML p.15/??

19 Some points to note ESC/Java forces one to specify some properties. If you understand the code, then these properties are obvious. But for larger programs this may not be the case! Erik Poll - JML p.15/??

20 Some points to note ESC/Java forces one to specify some properties. If you understand the code, then these properties are obvious. But for larger programs this may not be the case! If you have these properties documented, then understanding the code is easier. Erik Poll - JML p.15/??

21 ESC/Java vs runtime checking (cont.) For runtime assertion checking, we could choose what we specify, e.g. all, one, or none of the properties we have written for Bag. But for ESC/Java to accept a spec, we are forced to specify all properties (e.g. invariants, preconditions) that this spec relies on. Erik Poll - JML p.16/??

22 Like most tools, ESC/Java is Limitations of ESC/Java not complete: it may complain about a correct spec not sound: it may fail to warn about an incorrect spec ESC/Java warns about many potential bugs, but not about all actual bugs. These are unavoidable concessions to main goal: pointing out lots of potential bugs quickly & completely automatically In practice ESC/Java is quite good at checking simple specs, e.g. ruling out any NullPointer- and IndexOutOfBoundsExceptions Erik Poll - JML p.17/??