Jump to: navigation, search

Difference between revisions of "JDT Core/Null Analysis"

(Compiler configuration explained)
m (Actual Strategy in the JDT)
(36 intermediate revisions by 2 users not shown)
Line 11: Line 11:
 
}
 
}
 
</css>
 
</css>
This page discusses a proposed improvement in the static null analysis of the JDT compiler.
+
This page describes ongoing work on improving the static null analysis of the JDT compiler.
  
See also {{bug|186342}}.
+
The initial master bug for this work was  {{FixedBug|186342}}, this part has been released for Eclipse Juno (JDT 3.8).
  
  
Line 21: Line 21:
 
null checks etc.
 
null checks etc.
  
However, the current analysis is restricted to flow analysis '''within one method'''.
+
However, the analysis in JDT &le; 3.7 is restricted to flow analysis '''within one method'''.
 
No assumptions can be made about
 
No assumptions can be made about
 
* arguments flowing into a method
 
* arguments flowing into a method
Line 37: Line 37:
 
but having something like this in the JDT compiler should show two benefits:
 
but having something like this in the JDT compiler should show two benefits:
 
* feedback is more immediate and it is available for all JDT users without installing more software
 
* feedback is more immediate and it is available for all JDT users without installing more software
* analysis might be more precise than existing tools, because the actual flow analysis in the JDT compiler is already pretty strong (unproven claim).
+
* analysis might be more precise than some existing tools provide, because the actual flow analysis in the JDT compiler is already pretty strong (unproven claim).
  
A preparatory discussion of the design space can be found here: [[/Brainstorming]].
 
  
 +
[[Image:Video.png]] See also the recording of this [http://eclipsecon.org/europe2011/ ECE 2011] [http://eclipsecon.org/europe2011/sessions/bye-bye-npe session]: '''[http://www.fosslc.org/drupal/content/bye-bye-npe Bye, bye, NPE]'''
  
 
==Actual Strategy in the JDT==
 
==Actual Strategy in the JDT==
  
'''Disclaimer''': this is work in progress. No promise is made that this particular feature will be part of any particular release of the JDT.
+
{{note|Work in progress|For Eclipse Juno (JDT 3.8) we released a first version of this feature. The implemented features are complete and tested, but further improvements are still in the pipe line (see [[#Status]]).}}
  
By default the JDT does not support inter-procedural null analysis, however, a prototype exists allowing the JDT to be configured to use annotations for null contracts across method boundaries. The prototypical implementation is currently
+
By default the JDT does not support inter-procedural null analysis, however, starting with 3.8 the JDT can be configured to use annotations for extended null checking.
based on the [[:Category:OTEquinox|OT/Equinox]] technology for better maintainability at this stage. This particular prototype is known to have negative impact on the compiler ''performance'', which is however no indication about how the final in-place implementation will perform.
+
  
===Installing the prototype===
+
Up-to-date documentation for the annotation-based null analysis and its new configuration options can be found in the Eclipse help (Eclipse 3.8 and greater):
 +
* '''Java development user guide'''
 +
** '''Reference > Preferences > Java > Compiler > Errors/Warnings'''
 +
**: scroll down to '''Null analysis''' -- [http://help.eclipse.org/topic/org.eclipse.jdt.doc.user/reference/preferences/java/compiler/ref-preferences-errors-warnings.htm read online]
 +
** '''Tasks > Improving Java code quality > Using null annotations'''
 +
**: [http://help.eclipse.org/topic/org.eclipse.jdt.doc.user/tasks/task-using_null_annotations.htm?cp=1_3_9_0 read online]
  
* Get an Eclipse SDK &ge; 3.7M5
+
===Specifying nullness===
** A build &ge; 20110226 is required when using null annotations in <code>package-info.java</code>
+
** Some of the operations below (multi quickfixes) require lots of memory, you may want to add s.t. like <code>-Xmx800m</code> already now
+
* Enter this update URL:
+
** http://download.eclipse.org/objectteams/updates/contrib
+
* Select and install these features:
+
** JDT Null Annotation Checker (Incubation)
+
** Object Teams Equinox Integration (Incubation)
+
  
The code is hosted at
+
Null annotations in method signatures can be interpreted as [[/Null Contracts|null contracts]], however, a more general approach considers null annotations as an extension of the type system. Eventually - that is once JSR 308 can be used - all type references should either include or exclude null, which allows for complete checking of any possible dereferencing of null. In other words, a fully annotated program which passes the type checker will never raise an NPE at runtime.
: svn://dev.eclipse.org/svnroot/tools/org.eclipse.objectteams/trunk/plugins/org.eclipse.objectteams.jdt.nullity
+
  
Browse it at
+
To achieve this guarantee two annotations are used. The specific annotations types can be selected as a preference, but the following defaults are provided:
: http://dev.eclipse.org/viewcvs/viewvc.cgi/trunk/plugins/org.eclipse.objectteams.jdt.nullity/?root=TOOLS_OBJECTTEAMS
+
* [http://git.eclipse.org/c/jdt/eclipse.jdt.core.git/tree/org.eclipse.jdt.annotation/src/org/eclipse/jdt/annotation/NonNull.java org.eclipse.jdt.annotation.NonNull]
 +
* [http://git.eclipse.org/c/jdt/eclipse.jdt.core.git/tree/org.eclipse.jdt.annotation/src/org/eclipse/jdt/annotation/Nullable.java org.eclipse.jdt.annotation.Nullable]
  
===Example usage===
+
For any variable who's type is annotated with @NonNull (or the configured equivalent) the following rules apply:
In order to try the prototype against any existing Java project the following steps should help (I tried it using the JDT/Core as an example):
+
* It is illegal to bind null or a value that can be null to the variable. (For fields and local variables this applies to initialization and assignments, for method argument binding a value means to pass an actual argument in a method call).
 +
* It is legal and safe to dereference such a variable for accessing a field or a method of the bound object.
  
* Prepare the following compiler configuration
+
For any variable who's type is annotated with @Nullable (or the configured equivalent) the following rules apply:
** enable compliance 1.5 or higher
+
* It is legal to bind null or a value that can be null to the variable (see details above).
** enable all null-related warnings
+
* It is illegal to dereference such a variable for either field or method access.
** disable warnings regarding generics if the project doesn't use generics (don't spam the problems view)
+
  
* Since there isn't yet any UI for the new compiler preferences the following lines should be added manually into <code>.settings/org.eclipse.jdt.core.prefs</code>
+
The above rules imply that the value from a @NonNull variable can be bound to a variable annotated with @Nullable, but the opposite direction is generally illegal.
org.eclipse.jdt.core.compiler.annotation.nullable=annotations.Nullable
+
Only after an explicit null check can a @Nullable variable be treated as being @NonNull for the sake of binding to another @NonNull variable or for dereferencing. For fields the situation is actually more complex &mdash; please read [http://help.eclipse.org/kepler/topic/org.eclipse.jdt.doc.user/tasks/task-using_null_annotations.htm?cp=1_3_9_0_4#fields "The case of fields"].
org.eclipse.jdt.core.compiler.annotation.nonnull=annotations.NonNull
+
org.eclipse.jdt.core.compiler.annotation.nulldefault=nonnull
+
:* the first two options specify the fully qualified names of those annotation types to be used for null contracts.
+
:* the third option tells the compiler that any types that have no explicit null contract should be assumed as non-null (applies to method parameters and method return type).
+
  
* Create the annotation types specified above like this (in their correct package):
+
For interaction with inheritance see [[/Null Contracts#Null Contract Inheritance|Null Contract Inheritance]].
<source lang="java">
+
import static java.lang.annotation.ElementType.*;
+
import java.lang.annotation.*;
+
  
@Retention(RetentionPolicy.CLASS)
+
===Usage===
@Target({METHOD,PARAMETER,LOCAL_VARIABLE})
+
In order to try the new analysis against any existing Java project the following steps should help:
public @interface Nullable { }
+
</source>
+
<source lang="java">
+
import static java.lang.annotation.ElementType.*;
+
import java.lang.annotation.*;
+
  
@Retention(RetentionPolicy.CLASS)
+
* Open the compiler preferences for your project:
@Target({METHOD,PARAMETER,LOCAL_VARIABLE})
+
:* Ensure compliance is 1.5 or higher
public @interface NonNull { }
+
:* Find the section '''Null analysis''' and select '''Enable annotation-based null analysis'''
</source>
+
::[[Image:annotation-based-null-analysis.png]]
 +
:* You will be prompted to update the severity of some null-related problems, this is recommended.
 +
* Apply any of the annotations <code>@NonNull</code>, <code>@Nullable</code> or <code>@NonNullByDefault</code> in your code.
 +
** The annotation will be unresolvable at first, but a quick fix is offered to update the project setup:
 +
*: (see also: [http://help.eclipse.org/topic/org.eclipse.jdt.doc.user/tasks/task-using_null_annotations.htm&cp=1_3_9_0_2&anchor=buildpath_setup Help: Setup of the build path])
 +
*:* '''Copy library with default annotations to build path''' (plain Java projects), ''or'':
 +
*:* '''Add library with default annotations to build path''' (Plug-in projects).
 +
* Define <code>@NonNull</code> as the default at the granularity of your choice (package/type):
 +
** '''package''': add a file <code>package-info.java</code> with contents like this:
 +
**: <code>@NonNullByDefault package org.my.pack.age;</code>
 +
** '''type''': add <code>@NonNullByDefault</code> to the type declaration.
 +
* At this point you should see plenty of new errors and warnings
  
* Build (Project > Clean)
+
{{tip|Hints:|Setting the default to <code>@NonNull</code> is the recommended option for new projects, but for existing projects this could require a major clean-up in terms of inserting explicit <code>@Nullable</code> annotations in many locations. Experience shows, that this is a non-trivial task since in existing code the original intention, which parameter/return value should be allowed to be null, is usually blurred. Here starting with no default but adding individual annotations where the intention is clear will cause less disruption. This incremental approach should be seen as a long-term yet low effort task, not only for getting rid of NPEs but also for sorting the responsibilities in the code, which may have eroded over time.<br>For more hints on adpoting these annotations see [[JDT Core/Null Analysis/Adopting Null Annotations|Adoption Null Annotations]].}}
** At this point you should see plenty of new errors and warnings
+
  
 
====Cleaning up====
 
====Cleaning up====
The sheer number of new problems may look intimidating but that's where quickfixes will come to the rescue.
+
When applying the new analysis to a big existing project, the sheer number of new problems may look intimidating but that's where quick fixes will come to the rescue.
 
Currently the following problems offer a quickfix:
 
Currently the following problems offer a quickfix:
  
* <div class="b"><div class="hover">[[Image:Quickfix_error_obj.gif]] Null contract violation: returning null from a method declared as @NonNull</div></div>
+
* <div class="b"><div class="hover">[[Image:Quickfix_error_obj.gif]] Null type mismatch: required '@NonNull Foo' but the provided value is null</div></div>
* <div class="b"><div class="hover">[[Image:Quickfix_error_obj.gif]] Null contract violation: return value can be null but method is declared as @NonNull</div></div>
+
* <div class="b"><div class="hover">[[Image:Quickfix_error_obj.gif]] Null type mismatch: required '@NonNull Foo' but the provided value is specified as @Nullable</div></div> ''not in JDT 3.8.0 - see {{bug|337977#c19}}
 +
* <div class="b"><div class="hover">[[Image:Quickfix_error_obj.gif]] Null type mismatch: required '@NonNull Foo' but the provided value is inferred as @Nullable</div></div>
 +
* <div class="b"><div class="hover">[[Image:Quickfix_warning_obj.gif]] Null type safety: The expression of type Foo needs unchecked conversion to conform to '@NonNull Foo'</div></div>
 +
*: '''Fixable for these locations: return statements''':
 
*: Note that the mentioned @NonNull declaration may be implicit via an applicable default
 
*: Note that the mentioned @NonNull declaration may be implicit via an applicable default
*: In the second case use only with care: the compiler has no clear indication if @Nullable was actually intended or not
+
*: In cases 3) and 4) use only with care: the compiler has no clear indication if @Nullable was actually intended or not
 
*: The fix is:
 
*: The fix is:
 
*: <div class="b"><div class="fix">[[Image:Correction_change.gif]] Declare method return as @Nullable</div></div>
 
*: <div class="b"><div class="fix">[[Image:Correction_change.gif]] Declare method return as @Nullable</div></div>
* <div class="b"><div class="hover">[[Image:Quickfix_error_obj.gif]] Redundant null check: The variable foo cannot be null at this location</div></div>
+
* <div class="b"><div class="hover">[[Image:Quickfix_error_obj.gif]] Null comparison always yields false: The variable x is specified as @NonNull</div></div>
* <div class="b"><div class="hover">[[Image:Quickfix_error_obj.gif]] Null comparison always yields false: The variable bar cannot be null at this location</div></div>
+
* <div class="b"><div class="hover">[[Image:Quickfix_error_obj.gif]] Redundant null check: The variable x is specified as @NonNull</div></div>
*: Quickfix only applies to those occurrences that concern a method parameter
+
*: '''Fixable for these locations: null check for a method parameter'''
 
*: The fix is:
 
*: The fix is:
 
*: <div class="b"><div class="fix">[[Image:Correction_change.gif]] Declare method parameter as @Nullable</div></div>
 
*: <div class="b"><div class="fix">[[Image:Correction_change.gif]] Declare method parameter as @Nullable</div></div>
* <div class="b"><div class="hover">[[Image:Quickfix_error_obj.gif]] Cannot relax null contract for method return, inherited method from T is declared as @NonNull</div></div>
+
*: Otherwise a null check may indeed be unnecessary and should be deleted.
 +
* <div class="b"><div class="hover">[[Image:Quickfix_error_obj.gif]] The return type is incompatible with the @NonNull return from SuperFoo.foo()</div></div>
 +
*: '''Location: declaration of an overriding method'''
 
*: Note again that the mentioned @NonNull declaration may be due to a default.
 
*: Note again that the mentioned @NonNull declaration may be due to a default.
*: The fix is:
+
*: Possible fixes are:
*: <div class="b"><div class="fix">[[Image:Correction_change.gif]] Adjust overridden method from T, mark as returning @Nullable</div></div>
+
*: <div class="b"><div class="fix">[[Image:Correction_change.gif]] Change return type of foo(..) to '@NonNull'</div></div>
* <div class="b"><div class="hover">[[Image:Quickfix_error_obj.gif]] Cannot tighten null contract for parameter p, inherited method from T declares this parameter as @Nullable</div></div>
+
*: <div class="b"><div class="fix">[[Image:Correction_change.gif]] Change return type of overridden foo(..) to '@Nullable'</div></div>
* <div class="b"><div class="hover">[[Image:Quickfix_error_obj.gif]] Cannot tighten null contract for parameter p, inherited method from T does not constrain this parameter.</div></div>
+
* <div class="b"><div class="hover">[[Image:Quickfix_error_obj.gif]] Illegal redefinition of parameter a, inherited method from SuperFoo declares this parameter as @Nullable</div></div>
 +
* <div class="b"><div class="hover">[[Image:Quickfix_error_obj.gif]] Illegal redefinition of parameter a, inherited method from SuperFoo does not constrain this parameter.</div></div>
 +
*: '''Location: Parameter declaration of an overriding method'''
 
*: The second form occurs when no null default applies at the scope of the super method.
 
*: The second form occurs when no null default applies at the scope of the super method.
*: The fix is:
+
*: Possible fixes are:
*: <div class="b"><div class="fix">[[Image:Correction_change.gif]] Adjust overridden method from T, mark parameter as @NonNull</div></div>
+
*: <div class="b"><div class="fix">[[Image:Correction_change.gif]] Change parameter type to '@Nullable'</div></div>
 +
*: <div class="b"><div class="fix">[[Image:Correction_change.gif]] Change parameter type in overridden 'foo(..)' to '@NonNull'</div></div>
 +
* <div class="b"><div class="hover">[[Image:Quickfix_error_obj.gif]] Missing non-null annotation: inherited method from SuperClass declares this parameter as @NonNull</div></div>
 +
* <div class="b"><div class="hover">[[Image:Quickfix_error_obj.gif]] Missing nullable annotation: inherited method from SuperClass declares this parameter as @Nullable</div></div>
 +
*: '''Location: Parameter declaration of an overriding method'''
 +
*: Quick fix is either of:
 +
*: <div class="b"><div class="fix">[[Image:Correction_change.gif]] Change parameter type to @NonNull</div></div>
 +
*: <div class="b"><div class="fix">[[Image:Correction_change.gif]] Change parameter type to @Nullable</div></div>
  
These quickfixes can be applied...
+
These quick fixes can be applied...
 
* individually (Ctrl-1)
 
* individually (Ctrl-1)
 
* all occurrences per file (via the hover)
 
* all occurrences per file (via the hover)
 
* all occurrences (via context menu in the Problems view)
 
* all occurrences (via context menu in the Problems view)
Note, that some quickfixes require to modify another compilation unit (file) than the one
+
Note, that some quick fixes require to modify another compilation unit (file) than the one
 
where the problem was observed. For these quickfixes the current implementation doesn't
 
where the problem was observed. For these quickfixes the current implementation doesn't
 
support fixing several equal issues in bulk (for the technical background see
 
support fixing several equal issues in bulk (for the technical background see
 
{{bug|337977}}).
 
{{bug|337977}}).
 
===Compiler configuration explained===
 
By default the JDT does not recognize any null annotations but it can be configured to do so.
 
For this purpose these options are proposed:
 
::''Previous versions of this proposal contained ''emulation'' and ''default import'' of annotation types, which may, however, not be supported by the final solution. See older versions of this page for details (&le; 20110226).''
 
# '''Enable annotation based null analysis''' (enabled/disabled per project or workspace global).
 
#: ''This is the master switch for everything described in this wiki page''.
 
#: The built-in default for this option is:
 
#:: <code>org.eclipse.jdt.core.compiler.annotation.nullanalysis=disabled</code>
 
# Specify the '''names of annotation types''' to be used for marking nullable vs. nonnull types as well as for defining the default per type or per package.
 
#: ''Use this if you want to achieve compatibility with annotations defined by some other tool (or a future standard, should one be defined eventually).''
 
#: The built-in default values for these options are:
 
#:: <code>org.eclipse.jdt.core.compiler.annotation.nonnull=org.eclipse.jdt.annotation.NonNull</code>
 
#:: <code>org.eclipse.jdt.core.compiler.annotation.nullable=org.eclipse.jdt.annotation.Nullable</code>
 
#:: <code>org.eclipse.jdt.core.compiler.annotation.nonnullbydefault=org.eclipse.jdt.annotation.NonNullByDefault</code>
 
#:: <code>org.eclipse.jdt.core.compiler.annotation.nullablebydefault=org.eclipse.jdt.annotation.NullableByDefault</code>
 
# Define a '''global default''' for un-annotated entities (nonnull/nullable/''unspecified'')
 
#: ''Use this if your code has a bias to either @NonNull or @Nullable. By defining a global default you save the effort of adding annotations to the majority of locations.''
 
#: There is no built-in default for this option. The option name is
 
#:: <code>org.eclipse.jdt.core.compiler.annotation.nulldefault</code>
 
 
It is the user's responsibility to make the required annotation types available on the build path.
 
 
===Null Contracts===
 
Once properly configured the JDT compiler supports specification and checking of null contracts. Each part of a null contract implies an obligation for one side and a guarantee for the other side.
 
 
====Method Parameters====
 
 
When a method parameter is specified as '''nullable''' this defines the <font color="red">obligation</font> for the '''method implementation''' to cope with null values without throwing NPE. '''Clients''' of such a method enjoy the <font color="green">guarantee</font> that it is safe to call the method with a null value for the given parameter.
 
 
When a method parameter is specified as '''nonnull''' all '''clients''' are <font color="red">obliged</font> to ensure that null will never be passed as the value for this parameter. Thus the '''method implementation''' may rely on the <font color="green">guarantee</font> that null will never occur as the value for this parameter.
 
 
====Method Returns====
 
The situation is reversed for method returns. All four cases are summarized by the following table:
 
 
{|border=1
 
! !! caller !! method implementation
 
|-
 
| nullabel parameter
 
| <font color="green">may safely pass null without checking</font>
 
| <font color="red">must check before dereference</font>
 
|-
 
| nonnull parameter 
 
| <font color="red">must not pass null  </font>
 
| <font color="green">may use without checks</font>
 
|-
 
| nullable return   
 
| <font color="red">must check before dereference</font>
 
| <font color="green">can safely pass null</font>
 
|-
 
| nonnull return   
 
| <font color="green">may use without check</font>
 
| <font color="red">must not return null</font>
 
|-
 
|}
 
 
====Local Variables====
 
Null contracts can also be defined for local variables although this doesn't improve the
 
analysis by the compiler, because local variables can be fully analyzed without annotations, too. Here the main advantage of null annotations is in documenting intentions.
 
 
The following is an example of a program where all sides adhere to their respective part of the contract:
 
<source lang="java">
 
public class Supplier {
 
    // this method requires much but delivers little
 
    @Nullable String weakService (@NonNull String input, boolean selector) {
 
        if (selector)
 
            return input;
 
        else
 
            return null;
 
    }
 
    // this method requires nothing and delivers value
 
    @NonNull String strongService (@Nullable String input) {
 
        if (input == null)
 
          return "";
 
        else
 
          return input.toUpperCase();
 
    }
 
}
 
public class Client {
 
    void main(boolean selector) {
 
        Supplier supplier = new Supplier();
 
        @Nullable String value = supplier.weakService("OK", selector);
 
        @NonNull String result = supplier.strongService(value);
 
        System.out.println(result.toLowerCase());
 
    }
 
}
 
</source>
 
Notes:
 
* Althoug we know that toUpperCase() will never return null, the compiler does not know as long as <code>java.lang.String</code> does not specify null contracts. Therefor the compiler has to raise a warning that it has "insufficient nullness information" to guarantee contract adherence.
 
* Null annotations for the local variables are redundant here, as the nullness information can be fully derived from the variable's initialization (and no further assignments exist).
 
 
===Null Contract Inheritance===
 
A method that overrides or implements a corresponding method of a super type (class or interface) inherits the full null contract. Its implementation will thus be checked against
 
this inherited contract. For the sake of safe polymorphic calls,
 
null contracts must be inherited without modification, ''or'' be redefined in the following ways:
 
* A nonnull method parameter may be '''relaxed''' to a nullable parameter. The additional checks have to be performed in the body of the overriding method. Callers of the super type must still pass nonnull, while callers which are aware of the sub type may pass null.
 
* A nullable method return (or a return with no null annotation) may be '''tightened''' to a nonnull return. The additional checks must again be performed in the body of the overriding methods. Callers of the super type still have to check for null, whereas callers which are aware of the sub type may safely assume nonnull return values.
 
 
Any overrides that attempt to change a null contract in the opposite directions will raise a compile time error.
 
 
This explicitly implies that callers only need to inspect the null contract of the statically declared type of a call target to safely assume that all runtime call targets will adhere (at least) to the contract as specified in the statically declared type, even if the runtime type of the call target is any sub type of the declared type.
 
  
 
===Defaults at different levels===
 
===Defaults at different levels===
Line 243: Line 151:
 
* dereference without check.
 
* dereference without check.
  
The above mentioned preference (<code>org.eclipse.jdt.core.compiler.annotation.nulldefault</code>) allows to globally change this so that any declaration
+
To generally avoid these weak semantics you may want to declare that by default all types should be considered as nonnull.
to which no null annotation applies (directly or via inheritance) will be considered
+
as either nullable or nonnull, depending on that specific setting.
+
  
For more fine-grained control two additional annotations can be used. The qualified type
+
This is done using the annotation 'NonNullByDefault'. The qualified type
names of these annotations can be configured using these preferences:
+
name of this annotation can be configured using the preference "'NonNullByDefault' annotation".
org.eclipse.jdt.core.compiler.annotation.nullablebydefault
+
The built-in value for these preference is <code>org.eclipse.jdt.annotation.NonNullByDefault</code>.
org.eclipse.jdt.core.compiler.annotation.nonnullbydefault
+
  
The built-in values for these preferences are <code>org.eclipse.jdt.annotation.NullableByDefault</code> and <code>org.eclipse.jdt.annotation.NonNullByDefault</code>.
+
* This annotation takes an optional boolean parameter; when set to false this causes the annotation to ''cancel'' a default that may possible apply at the current location. This is useful when, e.g., sub-classing a legacy class without null annotation, where the sub-class sits in a place that would otherwise apply non-null as the default, which would make all overrides incompatible with inherited methods.
  
These annotations can be applied to any Java type or package and affect all  
+
This annotation can be applied to any package, Java type or method and affects all  
method returns and parameters with undefined null status within their scope.
+
method returns and parameters with undefined null status within their scope. (More locations will be supported in the future, but local variables are intentially unaffected by any default).
  
A default declared at an outer scope can be overridden by a different default at an
+
==Status==
inner scope. However, no means are provided for canceling a default (as to re-establish
+
 
the original Java semantics).
+
===Done===
 
+
At the current point the following bugs are resolved - functionality is released with Eclipse Juno:
===Future===
+
* {{FixedBug|186342}}.- [[Image:Ok_green.gif]] [compiler][null] Using annotations for null checking
The following bugzillas address future improvements of the above strategy (order roughly by priority):
+
* {{FixedBug|334455}} - [[Image:Ok_green.gif]] UI for new preferences regarding null annotations (plus a dup: {{FixedBug|364815}}).
* {{bug|334455}} - UI for new preferences regarding null annotations
+
* {{FixedBug|334457}} - [[Image:Ok_green.gif]] [compiler][null] check compatibility of inherited null contracts
* {{bug|334457}} - [compiler][null] check compatibility of inherited null contracts
+
* {{FixedBug|331647}} - [[Image:Ok_green.gif]] [compiler][null] support flexible default mechanism for null-annotations
* {{bug|331647}} - [compiler][null] support flexible default mechanism for null-annotations
+
* {{FixedBug|365208}} - [[Image:Ok_green.gif]] [compiler][batch] command line options for annotation based null analysis
* {{bug|331651}} - [compiler][null] Support nullity profiles for libraries
+
* {{FixedBug|331649}} - [[Image:Ok_green.gif]] [compiler][null] consider null annotations for fields
* {{bug|331649}} - [compiler][null] consider null annotations for fields
+
* {{FixedBug|337977}} - [[Image:Ok_green.gif]] [quick fix] Add quickfixes for null annotations
 +
* {{FixedBug|388281}} - [[Image:Ok_green.gif]] [compiler][null] inheritance of null annotations as an option
 +
 
 +
===Work in Progress===
 +
* {{bug|392099}} - [[Image:Progress.gif]] [1.8][compiler][null] Apply null annotation on types for null analysis - requires Java 8
  
 +
===Future===
 +
The following bugzillas address future improvements of the above strategy:
 +
* {{bug|331651}} - [[Image:Glass.gif]] [compiler][null] Support nullity profiles for libraries
 +
* {{bug|414237}} - [[Image:Glass.gif]] [compiler][null] Support a @LazyNonNull annotation for fields
 
[[Category:JDT]]
 
[[Category:JDT]]

Revision as of 10:23, 8 October 2013

This page describes ongoing work on improving the static null analysis of the JDT compiler.

The initial master bug for this work was bug 186342, this part has been released for Eclipse Juno (JDT 3.8).


Introduction

The static analysis of the JDT compiler detects many potential programming problems related to the null-ness of variables: dereferencing a null value (-> NPE), redundant null checks etc.

However, the analysis in JDT ≤ 3.7 is restricted to flow analysis within one method. No assumptions can be made about

  • arguments flowing into a method
  • return values from method calls and
  • field reads.

In order to include these elements in the analysis one could either

  • use whole program analysis (very expensive - not feasible for a (incremental) compiler)
  • explicit contracts via an extended type system or annotations

The second option is well explored in research and some existing tools (like the Checker Framework, JML, FindBugs) already introduce specific annotations to this end.

One could argue that advanced analysis should be left to specialized tools but having something like this in the JDT compiler should show two benefits:

  • feedback is more immediate and it is available for all JDT users without installing more software
  • analysis might be more precise than some existing tools provide, because the actual flow analysis in the JDT compiler is already pretty strong (unproven claim).


Video.png See also the recording of this ECE 2011 session: Bye, bye, NPE

Actual Strategy in the JDT

Note.png
Work in progress
For Eclipse Juno (JDT 3.8) we released a first version of this feature. The implemented features are complete and tested, but further improvements are still in the pipe line (see #Status).


By default the JDT does not support inter-procedural null analysis, however, starting with 3.8 the JDT can be configured to use annotations for extended null checking.

Up-to-date documentation for the annotation-based null analysis and its new configuration options can be found in the Eclipse help (Eclipse 3.8 and greater):

  • Java development user guide
    • Reference > Preferences > Java > Compiler > Errors/Warnings
      scroll down to Null analysis -- read online
    • Tasks > Improving Java code quality > Using null annotations
      read online

Specifying nullness

Null annotations in method signatures can be interpreted as null contracts, however, a more general approach considers null annotations as an extension of the type system. Eventually - that is once JSR 308 can be used - all type references should either include or exclude null, which allows for complete checking of any possible dereferencing of null. In other words, a fully annotated program which passes the type checker will never raise an NPE at runtime.

To achieve this guarantee two annotations are used. The specific annotations types can be selected as a preference, but the following defaults are provided:

For any variable who's type is annotated with @NonNull (or the configured equivalent) the following rules apply:

  • It is illegal to bind null or a value that can be null to the variable. (For fields and local variables this applies to initialization and assignments, for method argument binding a value means to pass an actual argument in a method call).
  • It is legal and safe to dereference such a variable for accessing a field or a method of the bound object.

For any variable who's type is annotated with @Nullable (or the configured equivalent) the following rules apply:

  • It is legal to bind null or a value that can be null to the variable (see details above).
  • It is illegal to dereference such a variable for either field or method access.

The above rules imply that the value from a @NonNull variable can be bound to a variable annotated with @Nullable, but the opposite direction is generally illegal. Only after an explicit null check can a @Nullable variable be treated as being @NonNull for the sake of binding to another @NonNull variable or for dereferencing. For fields the situation is actually more complex — please read "The case of fields".

For interaction with inheritance see Null Contract Inheritance.

Usage

In order to try the new analysis against any existing Java project the following steps should help:

  • Open the compiler preferences for your project:
  • Ensure compliance is 1.5 or higher
  • Find the section Null analysis and select Enable annotation-based null analysis
Annotation-based-null-analysis.png
  • You will be prompted to update the severity of some null-related problems, this is recommended.
  • Apply any of the annotations @NonNull, @Nullable or @NonNullByDefault in your code.
    • The annotation will be unresolvable at first, but a quick fix is offered to update the project setup:
    (see also: Help: Setup of the build path)
    • Copy library with default annotations to build path (plain Java projects), or:
    • Add library with default annotations to build path (Plug-in projects).
  • Define @NonNull as the default at the granularity of your choice (package/type):
    • package: add a file package-info.java with contents like this:
      @NonNullByDefault package org.my.pack.age;
    • type: add @NonNullByDefault to the type declaration.
  • At this point you should see plenty of new errors and warnings
Idea.png
Hints:
Setting the default to @NonNull is the recommended option for new projects, but for existing projects this could require a major clean-up in terms of inserting explicit @Nullable annotations in many locations. Experience shows, that this is a non-trivial task since in existing code the original intention, which parameter/return value should be allowed to be null, is usually blurred. Here starting with no default but adding individual annotations where the intention is clear will cause less disruption. This incremental approach should be seen as a long-term yet low effort task, not only for getting rid of NPEs but also for sorting the responsibilities in the code, which may have eroded over time.
For more hints on adpoting these annotations see Adoption Null Annotations.


Cleaning up

When applying the new analysis to a big existing project, the sheer number of new problems may look intimidating but that's where quick fixes will come to the rescue. Currently the following problems offer a quickfix:

  • Quickfix error obj.gif Null type mismatch: required '@NonNull Foo' but the provided value is null
  • Quickfix error obj.gif Null type mismatch: required '@NonNull Foo' but the provided value is specified as @Nullable
    not in JDT 3.8.0 - see bug 337977#c19
  • Quickfix error obj.gif Null type mismatch: required '@NonNull Foo' but the provided value is inferred as @Nullable
  • Quickfix warning obj.gif Null type safety: The expression of type Foo needs unchecked conversion to conform to '@NonNull Foo'
    Fixable for these locations: return statements:
    Note that the mentioned @NonNull declaration may be implicit via an applicable default
    In cases 3) and 4) use only with care: the compiler has no clear indication if @Nullable was actually intended or not
    The fix is:
    Correction change.gif Declare method return as @Nullable
  • Quickfix error obj.gif Null comparison always yields false: The variable x is specified as @NonNull
  • Quickfix error obj.gif Redundant null check: The variable x is specified as @NonNull
    Fixable for these locations: null check for a method parameter
    The fix is:
    Correction change.gif Declare method parameter as @Nullable
    Otherwise a null check may indeed be unnecessary and should be deleted.
  • Quickfix error obj.gif The return type is incompatible with the @NonNull return from SuperFoo.foo()
    Location: declaration of an overriding method
    Note again that the mentioned @NonNull declaration may be due to a default.
    Possible fixes are:
    Correction change.gif Change return type of foo(..) to '@NonNull'
    Correction change.gif Change return type of overridden foo(..) to '@Nullable'
  • Quickfix error obj.gif Illegal redefinition of parameter a, inherited method from SuperFoo declares this parameter as @Nullable
  • Quickfix error obj.gif Illegal redefinition of parameter a, inherited method from SuperFoo does not constrain this parameter.
    Location: Parameter declaration of an overriding method
    The second form occurs when no null default applies at the scope of the super method.
    Possible fixes are:
    Correction change.gif Change parameter type to '@Nullable'
    Correction change.gif Change parameter type in overridden 'foo(..)' to '@NonNull'
  • Quickfix error obj.gif Missing non-null annotation: inherited method from SuperClass declares this parameter as @NonNull
  • Quickfix error obj.gif Missing nullable annotation: inherited method from SuperClass declares this parameter as @Nullable
    Location: Parameter declaration of an overriding method
    Quick fix is either of:
    Correction change.gif Change parameter type to @NonNull
    Correction change.gif Change parameter type to @Nullable

These quick fixes can be applied...

  • individually (Ctrl-1)
  • all occurrences per file (via the hover)
  • all occurrences (via context menu in the Problems view)

Note, that some quick fixes require to modify another compilation unit (file) than the one where the problem was observed. For these quickfixes the current implementation doesn't support fixing several equal issues in bulk (for the technical background see bug 337977).

Defaults at different levels

If no null annotations are used, the compiler uses the original Java semantics, where the following is legal for all variables of reference types:

  • assign null, and
  • dereference without check.

To generally avoid these weak semantics you may want to declare that by default all types should be considered as nonnull.

This is done using the annotation 'NonNullByDefault'. The qualified type name of this annotation can be configured using the preference "'NonNullByDefault' annotation". The built-in value for these preference is org.eclipse.jdt.annotation.NonNullByDefault.

  • This annotation takes an optional boolean parameter; when set to false this causes the annotation to cancel a default that may possible apply at the current location. This is useful when, e.g., sub-classing a legacy class without null annotation, where the sub-class sits in a place that would otherwise apply non-null as the default, which would make all overrides incompatible with inherited methods.

This annotation can be applied to any package, Java type or method and affects all method returns and parameters with undefined null status within their scope. (More locations will be supported in the future, but local variables are intentially unaffected by any default).

Status

Done

At the current point the following bugs are resolved - functionality is released with Eclipse Juno:

  • bug 186342.- Ok green.gif [compiler][null] Using annotations for null checking
  • bug 334455 - Ok green.gif UI for new preferences regarding null annotations (plus a dup: bug 364815).
  • bug 334457 - Ok green.gif [compiler][null] check compatibility of inherited null contracts
  • bug 331647 - Ok green.gif [compiler][null] support flexible default mechanism for null-annotations
  • bug 365208 - Ok green.gif [compiler][batch] command line options for annotation based null analysis
  • bug 331649 - Ok green.gif [compiler][null] consider null annotations for fields
  • bug 337977 - Ok green.gif [quick fix] Add quickfixes for null annotations
  • bug 388281 - Ok green.gif [compiler][null] inheritance of null annotations as an option

Work in Progress

  • bug 392099 - Progress.gif [1.8][compiler][null] Apply null annotation on types for null analysis - requires Java 8

Future

The following bugzillas address future improvements of the above strategy:

  • bug 331651 - Glass.gif [compiler][null] Support nullity profiles for libraries
  • bug 414237 - Glass.gif [compiler][null] Support a @LazyNonNull annotation for fields