# API Lint
Metalava can optionally run warn about various API problems based on the Android
API Council's guidelines, described in go/android-api-guidelines.
These rules are not always exact. For example, you should avoid using acronyms
in names; e.g. a method should be named handleUri, not handleURI. But what about
getZOrder? This is probably better than getZorder, but metalava can't tell
without consulting a dictionary.
Therefore, there are cases where you want to say "ok, that's good advice in
general, but wrong here". In order to avoid having this warningshow up again
and again, there are two ways to mark an issue such that it is no longer
flagged.
(Note that metalava will not report issues on classes, methods and fields that
are deprecated since these are presumably already known to be bad and are already
discouraged.)
## Suppressing with @Suppress
Next to an error message, metalava will include the issue id. For example,
here's a sample error message:
src/android/pkg/MyStringImpl.java:3: error: Don't expose your implementation details: MyStringImpl ends with Impl [EndsWithImpl]
Here the id is "EndsWithImpl". You can suppress this with the @SuppressLint
annotation:
...
import android.annotation.SuppressLint;
...
@SuppressLint("EndsWithImpl")
public class MyStringImpl {
...
## Suppressing with baselines
Metalava also has support for "baselines", which are files which record all the
current warnings and errors in the codebase. When metalava runs, it looks up the
baseline to see if a given issue is already listed in the baseline, and if so,
it is silently ignored.
You can pass a flag to metalava ("--update-baseline") to tell it to update the
baseline files with any new errors it comes across instead of reporting
them. With soong, if you specify the baseline explicitly, like this:
--- a/Android.bp
+++ b/Android.bp
@@ -1678,6 +1678,7 @@ droidstubs {
},
api_lint: true,
==> baseline_filename: "api/baseline.txt", <==
jdiff_enabled: true,
}
then the build system will automatically supply `--update-baseline` on your
behalf to a generated file in the out/ folder, and if there's a failure metalava
will tell you where to find the updated baseline which you can then copy into
place:
...
93 new API lint issues were found. See tools/metalava/API-LINT.md for how to handle these.
************************************************************
Your API changes are triggering API Lint warnings or errors.
To make these errors go away, you have two choices:
1. You can suppress the errors with @SuppressLint("<id>")
2. You can update the baseline by executing the following
command:
cp \
out/soong/.intermediates/frameworks/base/system-api-stubs-docs/android_common/api/system-baseline.txt \
frameworks/base/api/system-baseline.txt
To submit the revised baseline.txt to the main Android
repository, you will need approval.
************************************************************
Then re-run the build and you should now see diffs to the baseline file; git
diff to make sure you're really only marking the issues you intended to include.
$ git status
...
Untracked files:
(use "git add <file>..." to include in what will be committed)
baseline.txt
## Copying File Manually
In the near future the build system will not allow source files to be modified
by the build. At that point you'll need to manually copy in the file instead.
During the build, before failing, metalava will emit a message like this:
...
metalava wrote updated baseline to out/something/baseline.txt
...
At that point you can copy the file to where you need:
```sh
$ cp out/something/baseline.txt frameworks/base/api/baseline.txt
```
## Existing Issues
You can view the exact set of existing issues (current APIs that get flagged by
API lint) by inspecting the baseline files in the source tree, but to get a
sense of the types of issues that are more likely to have a false positive,
here's the existing distribution as of early 2019:
Count Issue Id Rule Severity
--------------------------------------------------
932 ProtectedMember M7 error
321 OnNameExpected warning
288 ArrayReturn warning
269 ActionValue C4 error
266 NoByteOrShort FW12 warning
221 ExecutorRegistration L1 warning
211 AllUpper C2 error
206 GetterSetterNames M6 error
185 BannedThrow error
172 SamShouldBeLast warning
159 NoClone error
159 ParcelNotFinal FW8 error
119 NotCloseable warning
105 ListenerLast M3 warning
81 ConcreteCollection CL2 error
78 StaticUtils error
76 IntentName C3 error
74 VisiblySynchronized M5 error
72 GenericException S1 error
65 KotlinOperator warning
57 AcronymName S1 warning
55 ParcelCreator FW3 error
54 ParcelConstructor FW3 error
53 UseIcu warning
48 Enum F5 error
41 RethrowRemoteException FW9 error
37 AutoBoxing M11 error
35 StreamFiles M10 warning
28 IntentBuilderName FW1 warning
27 ServiceName C4 error
26 ListenerInterface L1 error
25 ContextFirst M3 error
25 InterfaceConstant C4 error
24 CallbackInterface CL3 error
24 RegistrationName L3 error
23 IllegalStateException S1 warning
22 EqualsAndHashCode M8 error
22 PackageLayering FW6 warning
18 MinMaxConstant C8 warning
18 SingletonConstructor error
17 MethodNameUnits error
15 MissingBuild warning
15 UserHandleName warning
14 UserHandle warning
13 ResourceFieldName error
12 ManagerLookup error
11 ManagerConstructor error
9 CallbackMethodName L1 error
9 ParcelableList warning
8 CallbackName L1 warning
7 HeavyBitSet error
7 ResourceValueFieldName C7 error
6 CompileTimeConstant error
6 SetterReturnsThis M4 warning
4 EndsWithImpl error
4 TopLevelBuilder warning
4 UseParcelFileDescriptor FW11 error
3 MentionsGoogle error
3 StartWithLower S1 error
2 AbstractInner warning
2 BuilderSetStyle warning
2 OverlappingConstants C1 warning
2 PairedRegistration L2 error
2 SingularCallback L1 error
2 StartWithUpper S1 error
1 ContextNameSuffix C4 error
1 KotlinKeyword error
--------------------------------------------------
4902
(This is generated when metalava is invoked with both --verbose and --update-baseline.)