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Null Analysis Options
Note
This page served as background material in the discussion how to design null annotations and their interpretation. One particular feature that resulted from this discussion is "syntactic null analysis for fields", see also bug 383368. Also the quick fix for extracting a nullable field to a local variable before dereference was developed in this context. In addition, the concept of "@LazyNonNull" (called "EventuallyNonNull" in the discussion below) is still on the agenda, see bug 414237.
The analysis of possible null pointer exceptions performed by the Eclipse Java Compiler is powerful, but the full power may result in a large number of errors and warnings. Not all projects can afford addressing all these issues (at once).
This page discusses the problem from two different directions:
- On the road towards a fully checked, NPE-free program, what are the risks needing to be addressed before the solution can be considered complete, i.e., fully safe?
- Should a more lenient analysis be applied to reduce the number
of warnings perceived as uninteresting?
- What are the risks that more relaxed rules would introduce?
- What are the patterns of coding that (some) users would like to see accepted without a warning?
- What assumptions need to be maintained to render these patterns safe?
If annotation based null analysis applies strict pessimistic rules, a full guarantee of absence of NPE is only threatened by two risks:
- specification of nullness is either incomplete or incompletely checked
- checked null specifications are bypassed at runtime
Aside from these an approach that strictly views null annotations as an extension of the type system is fully safe.
If you start with an existing project and only turn on annotation based null analysis, you will not notice any difference. This is because the analysis implicitly supports three kinds of reference types:
- @NonNull types
- @Nullable types and
- types with unspecified nullness (legacy types).
So before you actually add annotations to your methods and fields, all types will be considered as legacy types and so the unsafe semantics of Java apply where both assigning null and dereferencing are legal.
This path is obvious: add just those annotations where you are certain about the design intent and after each annotation added address the problems reported by the compiler.
Instead of going in tiny little steps you may want to do jumps of some size (cf. bug 331647:
- annotate a method as @NonNullByDefault to affect all its parameters and its return
- annotate a type as @NonNullByDefault to affect all its methods and fieds
- annotate a package as @NonNullByDefault (using package-info.java) to affect all its types
- define a global policy that all packages should be @NonNullByDefault Unfortunately, a global default cannot directly be established but only via the indirection of package level defaults (see bug 366063 for background).
A particular problem arises when a type affected by @NonNullByDefault is a subtype of a legacy type with legacy signatures. It is illegal to override a method with legacy parameters by a method with @NonNull parameters. Specifying @Nullable for all parameters in such an overriding method may be too pessimistic, forcing the method body to do more null checks than actually useful.
For this situation a parameter has been added to the default annotation. Declaring a type that inherits from a legacy type with @NonNullByDefault({}) for v2 Java 8 type null annotations respectively NonNullByDefault(false) for v1 cancels the applicable default.
Every project depends on third party code, which it doesn't control, so adding null annotations is not directly possible.
To address this issue, the Eclipse Java Compiler should support nullity profiles, aka external annotations: separate files that capture the factual null contracts of all API methods and fields contained in a given library.
Support for this feature is planned for version 3.9, see bug 331651.
In order to ensure that no unchecked legacy types are used in an application, these things must be ensured:
- @NonNullByDefault is the globally enforced default
- @NonNullByDefault(false) (v1) / @NonNullByDefault({}) (v2, type) is never used
- all libraries come with API-complete nullity profiles
Each of these steps brings a project closer to the safety guarantees of complete analysis.
TODO: Issue a configurable warning when @NonNullByDefault({} / false) is used.
TODO: Add a note to bug 331651 for checking complete coverage of referenced libraries by available nullity profiles.
Any guarantees given by the analysis can still be bypassed at runtime if one of the following is applied:
- runtime reflection beyond pure introspection, i.e, whenever reflective field access or method invocation is involved.
- bytecode manipulation, i.e., whenever the bytes being executed by the VM are not identical to the bytes produced at the time when null analysis was performed.
These risks are typically considered outside the scope of linguistic means, developers know about these issues at a general level and don't expect a static analysis to consider these.
New Projects
Readers interested in using null annotations for new projects may safely stop reading here, because writing your code in a style that passes the strict null analysis is much cheaper than going through all the alternatives discussed below.
Some warnings/errors flagged by a strict analysis may seem counter intuitive. Some may actually be irrelevant in specific contexts.
Practical investigations showed that many of those "unwanted" warnings/errors relate to field access. When focusing on these, the most obvious approach at distinguishing interesting from uninteresting warnings is to apply some kind of flow analysis for fields (which the pessimistic approach strictly avoids).
When flow analysis is applied to fields the following reasons may cause unexpected NPE:
- side effects
- implicit control flows
- aliasing
- concurrency
Null analysis for the following kinds of fields is essentially unaffected by these risks:
- final fields
- @NonNull fields
For both kinds of fields only object initialization may produce unexpected results (yes, even Java's definite assignment rule can be circumvented). Once initialized their null status is either known or amenable to flow analysis.
TODO: more precise distinction for final fields: is static relevant? does initialization occur directly as part of the field declaration?
When considering flow analysis for fields another annotation becomes
highly relevant, let's call it @EventuallyNonNull (aka @LazyNonNull
etc.). This annotation has been proposed by different researchers to
establish monotonicity for a field, which means that a field can never
change from non-null to null, which makes fields with this annotation
amenable to flow analysis. The semantics is defined like this
- initially the field may be
null - any value assigned to the field must be provably non-null.
The simplest problem with flow analysis for fields results from side effects in methods:
class X {
@Nullable Object f;
void test() {
if (this.f != null) {
foo();
this.f.bar();
}
}
void foo() { /* arbitrary code here */ }
}The execution of foo() could potentially assign null to f thus
invalidating the above null check.
The problem of side effects is aggravated by the fact, that in Java not all control flows are visible in the source code. In particular, class loading may happen at unexpected points in time and my trigger additional behavior which can interfere with null analysis:
class X {
@Nullable Object f;
void test(Ojbect arg) {
if (this.f != null && Z.CONSTANT != arg) {
this.f.bar();
}
}
}The mere fact, that evaluating Z.CONSTANT needs to access class Z,
which may trigger any static initializers of Z, implies that this access
to a constant may invalidate the flow analysis for field this.f.
When applying flow analysis for checking nullness of fields the following snippet demonstrates how aliasing threatens the validity:
class X {
@Nullable Y f;
void test(X other) {
if (this.f != null) {
other.f = null;
this.f.bar(); // potential NPE
}
}
void breakIt() {
test(this); // definitely triggers the NPE
}
}Without further annotations an intra-procedural analysis cannot see that
the assignment to other.f affects the value of this.f.
Also concurrency can invalidate the null related guarantees of flow analysis for fields. In a concurrent application not even this snippet is safe:
if (this.f != null)
this.f.bar();Thus concurrency adds to the problems of side effects and aliasing, because null information from one expression may already be invalid when evaluating the very next expression.
Given the above risks, we can admit certain styles of code if a certain risk is considered irrelevant in a given context. The discussion will use this basic question:
class X {
@Nullable Object f;
void test() {
if (this.f != null) {
// what code is allowed here?
this.f.bar(); // want to be sure there's no NPE here
}
}
}What code can we admit at the designated location? What assumptions must a developer ensure to render that code valid?
The interesting property of fields declared as @EventuallyNonNull is,
that even the most unsafe examples shown below become safe. The
monotonicity property guarantees that the information from the null
check remains valid for the entire remainder of that particular flow,
i.e. here: until the end of the then-block, no matter what code comes
in between.
If the code between check and dereference contains any method call, we have the fragment used for demonstrating side effects:
class X {
@Nullable Object f;
void test() {
if (this.f != null) {
foo();
this.f.bar();
}
}
}It turns out that all three risks mentioned above are effective in this situations.
This requires to confidently state the assumption that one's application is free from side effects, aliasing and concurrency. In typical object-oriented code that assumption, however, is unfounded. Thus admitting any method call in this location lacks theoretical nor practical foundation.
class X {
@Nullable Object f;
void test(X other) {
if (this.f != null) {
other.f = null;
this.f.bar();
}
}
}When demonstrating the effects of aliasing we discussed that the above fragment is not OK.
Still similar patterns could be accepted, if both of the following hold:
- no other thread concurrently accesses the field f, and
- we can statically determine that a field f being assigned is different from the field being dereferenced afterwards.
For the latter assumption we could apply one of these strategies:
- full alias analysis (not realistic for the JDT)
- static approximation: ignore that different objects hold different incarnations of the same field, treat any assignment to a field as invalidating the analysis results for all objects of that class.
Using static approximation the above fragment would be considered as unsafe, because the assignment refers to the same field, although at runtime different incarnations (of different receiver instances 'this' and 'other') may be involved. This static approximation will thus flag a few locations that might be OK given more knowledge about the instances 'this' and 'others', but the guarantees given by this approach can only be broken by concurrency.
So far we have focused on field references of the shape this.f. Can
the previous discussion be extended to other shapes of field references?
When we want to apply the above to field references local.f, where
local is any local variable in scope, we have to ensure one of these:
-
localis declaredfinal, or - analysis discards all information for
local.fat any assignment tolocalitself.
So this is safe:
class X {
@Nullable Object f;
void test() {
X local = new X();
if (local.f != null) {
// code without method calls nor assignments to local
local.f.bar();
}
}
}while this is unsafe:
class X {
@Nullable Object f;
void test(X other) {
X local = new X();
if (local.f != null) {
local = other; // status of other.f is unknown
local.f.bar();
}
}
}This strategy is conceptually sound. It is a idiosyncrasy of the analysis engine applied inside the JDT, that implementation of this is far from trivial.
For field references that contain more than one dot (and also those
f.f2 where f is already a field reference, relative to the implicit
'this') more must be considered as illustrated by the following example:
class Y {
@Nullable Z f2;
}
class X {
Y f;
void test(X other, Y yippie) {
X local = other;
if (local.f.f2 != null) {
// either of the next three assigments invalidates the check result:
other.f = yippie;
local.f = yippie;
local = other;
local.f.f2.bar();
}
}
}At this point, the additional segment means we need to ensure that no
link in the chain from local to f2 breaks. Again two possible
strategies are possible:
- require all links in the chain to be final
- discard analysis results for
local.f.f2at every assignment that could affect one of these links.
Without full alias analysis the second strategy will amount to a static
approximation where, e.g., assignment to any f would trigger
discarding the analysis result.
While results for @Nullable fields are loosing more and more precision when relaxing the rules in one of the ways discussed above, we saw that @EventuallyNonNull fields can still support strong guarantees.
For this.f references to an @EventuallyNonNull field we can accept any
code between a check and a dereference.
For local.f references we only have to ensure that local is not
locally assigned a different value between check and dereference. If
that is given, we can again accept any code between a check and a
dereference.
For local.f.f2 references even to @EventuallyNonNull fields the
desired guarantee can be violated, as demonstrated by this example:
class Y {
@EventuallyNonNull Z f2;
}
class X {
Y f;
void test(X other, Y yippie) {
if (other.f.f2 != null) {
foo();
other.f.f2.bar();
}
}
void foo() {
this.f = new Y();
}
void breakIt() {
test(this, new Y());
}
}Due to the alias created between 'this' and 'other' (line 16), the assignment inside the execution of foo changes the meaning of 'other.f', thus the assumed knowledge about 'other.f.f2' is wrong, and no strategy short of full alias analysis can restore the desired guarantee.
Additionally, concurrency could also affect f thus invalidating the
safety guarantee.
This means the promise given by applying @EventuallyNonNull cannot be fulfilled when including multi-segment field references in flow analysis. Positively speaking: for analyzing a reference to an @EventuallyNonNull field, both of the following must hold:
- the first segment must be under the control of the current method (either 'this' or a final local or a local with discarding information at every assignment to this local), and
- if the field reference has intermediate links all these must be
final
Independent of the discussion above, the following special case can easily be detected by the compiler:
class X {
void test(X other) {
if (other.f.f2 != null) {
other.f.f2.bar();
}
}
}This is to say, the code could be considered as safe if a dereference directly follows a null check where both expressions are exactly identical references at the syntax level. Considering this style as safe only risks violation by concurrency.
For non-final @Nullable fields the simplest strategy for avoiding any nullness related risks is to pessimistically assume potential null at every read. This means for strict checking no flow analysis should be applied to @Nullable fields.
While this appears to be a very drastic restriction, the remedy is quite easy: before dereferencing a @Nullable field it has to be assigned to a local variable. Flow analysis is then safely applied to the local variable with no risk of side effects, aliasing nor concurrency, since local variables are not shared with any code locations that would be outside the scope of the analysis. I.e., the flow analysis can see everything it needs to consider regarding local variables.
Any flow analysis performs much better for local variables than for fields. Shifting all serious computation to local variables makes for much safer code. Whether or not a project can afford to fully enforce this strategy depends on many factors.
Next to the use of local variables also @EventuallyNonNull makes the code much more amenable to flow analysis, thus fully reliable analysis can be achieved even without the abundant use of local variables. If null checks directly against a field should be exploited using flow analysis, applying @EventuallyNonNull provides the biggest gain. However, the guarantees of this annotation are bounded by the kind of field references used, they don't hold for the general case of multi-segment references.
In situations where concurrency and aliases are considered as irrelevant some limited forms of flow analysis can also be applied to @Nullable fields. Here the kind of field reference only mildly affects the precision of the analysis, which is already very limited anyway. Perhaps a large portion in this domain is already covered by the syntactic special case proposed above.
Aside from the technical discussion of what assumptions support which guarantees, also the following questions deserve further investigation:
- How can gradual migration to a fully safe coding style fully based on local variables be supported. I.e., what intermediate levels are useful, what configuration options are needed and what warning messages best convey the vagueness/certainty of each issue?
- How can the above options be communicated to users? Is it OK to offer options like "perform flow analysis for fields", or should options be labeled as "consider aliasing", or "pessimistically consider concurrency for null analysis"?
- How can initialization be made safer? How relevant are these issues in practice? Which annotation / set of annotations is the best buy? Is support for several styles required?