Improving the java type system
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This document discusses improving the Java type system by adding behavioral assertions. It describes how assertions can be added for fields, methods, constructors, and on method entry/exit. Fields are handled by injecting code to track writes. Method assertions are added by wrapping the original method. Constructor assertions are added by inserting code before the body. Entry/exit assertions require injecting code at both the start and end of the method. The approach aims to not break original program functionality while checking assertions. Examples are provided for each case.
![ASSERTIONS ON METHOD ENTRY
We inject code at begining of method and at the exit.
It should return NPE!
public void m() {
// entry assertion
if(!(getAndClear() == null)) //...
temp = m$wrapped();
// value assertion
if(!(getAndClear() != null)) //...
return temp;
}
## As you can see (getAndClear() == null) expression returned true
[java] Exception in thread "main" java.lang.NullPointerException
[java] at ist.meic.pa.tests.Base.m$wrapped(Unknown Source)
[java] at ist.meic.pa.tests.Base.m(Unknown Source)
[java] ...
[java] The assertion getAndClear() != null is false](https://image.slidesharecdn.com/improving-20the-20java-20type-20system-130515093624-phpapp02/85/Improving-the-java-type-system-11-320.jpg)
Improving the java type system
- 1. IMPROVING THE JAVA TYPE SYSTEM João Loff - 56960 Alexandre Almeida - 64712 Tiago Aguiar - 64870
- 2. BEHAVIORS 180 LOC “CtBehavior represents a method, a constructor, or a static constructor” for (CtBehavior behavior : clazz.getDeclaredBehaviors()){ behavior.instrument(new ExprEditor() { ... }); if(behavior instanceof CtMethod) { ...} if(behavior instanceof CtConstructor) { ... } }
- 3. FIELD INITIALIZATION We start with: We end up with: @Assertion("true") int foo; foo++; static java.util.HashSet f$writes = new java.util.HashSet(); @Assertion("true") int foo; if(!f$writes.contains(foo_hash_code)) //... foo++ f$writes.add(foo_hash_code); if(!(true)) //...
- 4. FIELD INITIALIZATION How to we solve this? DOUBLE CODE INJECTION @Assertion("true") int foo; @Assertion("bar>foo") int bar; bar=1; foo++;
- 5. FIELD INITIALIZATION The first injection The second injection if(!f$writes.contains(bar_hash_code)) //... bar=1; f$writes.add(bar_hash_code); if(!(bar>foo)) //... if(!f$writes.contains(bar_hash_code)) //... bar=1; f$writes.add(bar_hash_code); if(!f$writes.contains(bar_hash_code)) //... if(!f$writes.contains(foo_hash_code)) //... if(!(bar>foo)) //... f$writes.add(bar_hash_code); if(!(bar>foo)) //...
- 6. METHOD ASSERTION This shouldn't return NPE! METHOD WRAPPING Object o = new Object(); public Object getAndClear() { return this.o = null; } @Assertion("getAndClear() != null") public void m() { this.o.toString(); }
- 7. METHOD ASSERTION We end up with something kinda like this: A "new" method that returns the result of the original one public void m() { // temp "is a" bytecode var temp = m$wrapped(); // now we check assertions if(!(getAndClear() != null)) //... // and now we return! return temp; } @Assertion("getAndClear() != null") public void m$wrapped() { this.o.toString(); }
- 8. METHOD ASSERTIONS Inherited Method Assertions We recursively inject code from parents assertions class Base { @Assertion("false") public int fooBar(int x) { ... } } public class Derived extends Base { @Override @Assertion("true") public int fooBar(int x) { ... } } public int fooBar(int x) { temp = fooBar$wrapped(x); if(!(false)) //... if(!(true)) //... return temp; }
- 9. CONSTRUCTOR ASSERTIONS Remember Behaviors? That was easy. if((behavior instanceof CtConstructor) && hasAssertion(behavior)){ CtConstructor constructor = (CtConstructor) behavior; constructor.insertBeforeBody( "if(!(" + getAssertionValues(constructor) + ")) //... " ); }
- 10. ASSERTIONS ON METHOD ENTRY We can't break original functionality! Previous example, now with entry assertion Object o = new Object(); public Object getAndClear() { return this.o = null; } @Assertion( value="getAndClear() != null", entry="getAndClear() == null" ) public void m() { this.o.toString(); }
- 11. ASSERTIONS ON METHOD ENTRY We inject code at begining of method and at the exit. It should return NPE! public void m() { // entry assertion if(!(getAndClear() == null)) //... temp = m$wrapped(); // value assertion if(!(getAndClear() != null)) //... return temp; } ## As you can see (getAndClear() == null) expression returned true [java] Exception in thread "main" java.lang.NullPointerException [java] at ist.meic.pa.tests.Base.m$wrapped(Unknown Source) [java] at ist.meic.pa.tests.Base.m(Unknown Source) [java] ... [java] The assertion getAndClear() != null is false
- 12. QUESTIONS?