README.md - platform/tools/metalava - Git at Google

 # Metalava

 Metalava is a metadata generator intended for JVM type projects. The main
 users of this tool are Android Platform and AndroidX libraries, however this
 tool also works on non-Android libraries.

 Metalava has many features related to API management. Some examples of the most
 commonly used ones are:

 * Allows extracting the API (into signature text files, into stub API files
   which in turn get compiled into android.jar, the Android SDK library) and
   more importantly to hide code intended to be implementation only, driven by
   javadoc comments like @hide, @doconly, @removed, etc, as well as various
   annotations.

 * Extracting source level annotations into external annotations file (such as
   the typedef annotations, which cannot be stored in the SDK as .class level
   annotations) to ship alongside the Android SDK and used by Android Lint.

 * Diffing versions of the API and determining whether a newer version is
   compatible with the older version. (See [COMPATIBILITY.md](COMPATIBILITY.md))

 ## Building and running

 To download the code and any dependencies required for building, see [DOWNLOADING.md](DOWNLOADING.md)

 To build:

     $ cd tools/metalava
     $ ./gradlew

 It puts build artifacts in `../../out/metalava/`.

 To run the metalava executable:

 ### Through Gradle

 To list all the options:

     $ ./gradlew run

 To run it with specific arguments:

     $ ./gradlew run --args="--api path/to/api.txt"

 ### Through distribution artifact

 First build it with:

     $ ./gradlew installDist

 Then run it with:

     $ ../../out/metalava/install/metalava/bin/metalava
                     _        _
      _ __ ___   ___| |_ __ _| | __ ___   ____ _
     | '_ ` _ \ / _ \ __/ _` | |/ _` \ \ / / _` |
     | | | | | |  __/ || (_| | | (_| |\ V / (_| |
     |_| |_| |_|\___|\__\__,_|_|\__,_| \_/ \__,_|

     metalava extracts metadata from source code to generate artifacts such as the
     signature files, the SDK stub files, external annotations etc.

     Usage: metalava <flags>

     Flags:

     --help                                This message.
     --quiet                               Only include vital output
     --verbose                             Include extra diagnostic output

     ...

 (*output truncated*)

 ## Features

 * Ability to read in an existing android.jar file instead of from source, which
   means we can regenerate signature files etc for older versions according to
   new formats (e.g. to fix past errors in doclava, such as annotation instance
   methods which were accidentally not included.)

 * Ability to merge in data (annotations etc) from external sources, such as
   IntelliJ external annotations data as well as signature files containing
   annotations. This isn't just merged at export time, it's merged at codebase
   load time such that it can be part of the API analysis.

 * Support for an updated signature file format (which is described in [FORMAT.md](FORMAT.md))

   * Address errors in the doclava1 format which for example was missing
     annotation class instance methods

   * Improve the signature format such that it for example labels enums "enum"
     instead of "abstract class extends java.lang.Enum", annotations as
     "@interface" instead of "abstract class extends java.lang.Annotation", sorts
     modifiers in the canonical modifier order, using "extends" instead of
     "implements" for the superclass of an interface, and many other similar
     tweaks outlined in the `Compatibility` class. (Metalava also allows (and
     ignores) block comments in the signature files.)

   * Add support for writing (and reading) annotations into the signature
     files. This is vital now that some of these annotations become part of the
     API contract (in particular nullness contracts, as well as parameter names
     and default values.)

   * Support for a "compact" nullness format -- one based on Kotlin's
     syntax. Since the goal is to have **all** API elements explicitly state
     their nullness contract, the signature files would very quickly become
     bloated with @NonNull and @Nullable annotations everywhere. So instead, the
     signature format now uses a suffix of `?` for nullable, `!` for not yet
     annotated, and nothing for non-null.

     Instead of

         method public java.lang.Double convert0(java.lang.Float);
         method @Nullable public java.lang.Double convert1(@NonNull java.lang.Float);

     we have

         method public java.lang.Double! convert0(java.lang.Float!);
         method public java.lang.Double? convert1(java.lang.Float);

   * Other compactness improvements: Skip packages in some cases both for export
     and reinsert during import. Specifically, drop "java.lang."  from package
     names such that you have

         method public void onUpdate(int, String);

     instead of

         method public void onUpdate(int, java.lang.String);

     Similarly, annotations (the ones considered part of the API; unknown
     annotations are not included in signature files) use just the simple name
     instead of the full package name, e.g. `@UiThread` instead of
     `@android.annotation.UiThread`.

   * Misc documentation handling; for example, it attempts to fix sentences that
     javadoc will mistreat, such as sentences that "end" with "e.g. ".  It also
     looks for various common typos and fixes those; here's a sample error
     message running metalava on master: Enhancing docs:

         frameworks/base/core/java/android/content/res/AssetManager.java:166: error: Replaced Kitkat with KitKat in documentation for Method android.content.res.AssetManager.getLocales() [Typo]
         frameworks/base/core/java/android/print/PrinterCapabilitiesInfo.java:122: error: Replaced Kitkat with KitKat in documentation for Method android.print.PrinterCapabilitiesInfo.Builder.setColorModes(int, int) [Typo]

 * Built-in support for injecting new annotations for use by the Kotlin compiler,
   not just nullness annotations found in the source code and annotations merged
   in from external sources, but also inferring whether nullness annotations have
   recently changed and if so marking them as @Migrate (which lets the Kotlin
   compiler treat errors in the user code as warnings instead of errors.)

 * Support for generating documentation into the stubs files (so we can run
   javadoc or [Dokka](https://github.com/Kotlin/dokka) on the stubs files instead
   of the source code). This means that the documentation tool itself does not
   need to be able to figure out which parts of the source code is included in
   the API and which one is implementation; it is simply handed the filtered API
   stub sources that include documentation.

 * Support for parsing Kotlin files. API files can now be implemented in Kotlin
   as well and metalava will parse and extract API information from them just as
   is done for Java files.

 * Like doclava1, metalava can diff two APIs and warn about API compatibility
   problems such as removing API elements. Metalava adds new warnings around
   nullness, such as attempting to change a nullness contract incompatibly
   (e.g. you can change a parameter from non null to nullable for final classes,
   but not versa).  It also lets you diff directly on a source tree; it does not
   require you to create two signature files to diff.

 * Consistent stubs: In doclava1, the code which iterated over the API and
   generated the signature files and generated the stubs had diverged, so there
   was some inconsistency. In metalava the stub files contain **exactly** the
   same signatures as in the signature files.

   (This turned out to be incredibly important; this revealed for example that
   StringBuilder.setLength(int) was missing from the API signatures since it is a
   public method inherited from a package protected super class, which the API
   extraction code in doclava1 missed, but accidentally included in the SDK
   anyway since it packages package private classes. Metalava strictly applies
   the exact same API as is listed in the signature files, and once this was
   hooked up to the build it immediately became apparent that it was missing
   important methods that should really be part of the API.)

 * API Lint: Metalava can optionally (with --api-lint) run a series of additional
   checks on the public API in the codebase and flag issues that are discouraged
   or forbidden by the Android API Council; there are currently around 80 checks.
   Some of these take advantage of looking at the source code which wasn't
   possible with the signature-file based Python version; for example, it looks
   inside method bodies to see if you're synchronizing on this or the current
   class, which is forbidden.

 * Baselines: Metalava can report all of its issues into a "baseline" file, which
   records the current set of issues. From that point forward, when metalava
   finds a problem, it will only be reported if it is not already in the
   baseline.  This lets you enforce new issues going forward without having to
   fix all existing violations. Periodically, as older issues are fixed, you can
   regenerate the baseline. For issues with some false positives, such as API
   Lint, being able to check in the set of accepted or verified false positives
   is quite important.

 * Metalava can generate reports about nullness annotation coverage (which helps
   target efforts since we plan to annotate the entire API). First, it can
   generate a raw count:

         Nullness Annotation Coverage Statistics:
         1279 out of 46900 methods were annotated (2%)
         2 out of 21683 fields were annotated (0%)
         2770 out of 47492 parameters were annotated (5%)

   More importantly, you can also point it to some existing compiled applications
   (.class or .jar files) and it will then measure the annotation coverage of the
   APIs used by those applications. This lets us target the most important APIs
   that are currently used by a corpus of apps and target our annotation efforts
   in a targeted way. For example, running the analysis on the current version of
   framework, and pointing it to the
   [Plaid](https://github.com/nickbutcher/plaid) app's compiled output with

       ... --annotation-coverage-of ~/plaid/app/build/intermediates/classes/debug

   This produces the following output:

     324 methods and fields were missing nullness annotations out of 650 total
     API references.  API nullness coverage is 50%

     ```
     | Qualified Class Name                                         |      Usage Count |
     |--------------------------------------------------------------|-----------------:|
     | android.os.Parcel                                            |              146 |
     | android.view.View                                            |              119 |
     | android.view.ViewPropertyAnimator                            |              114 |
     | android.content.Intent                                       |              104 |
     | android.graphics.Rect                                        |               79 |
     | android.content.Context                                      |               61 |
     | android.widget.TextView                                      |               53 |
     | android.transition.TransitionValues                          |               49 |
     | android.animation.Animator                                   |               34 |
     | android.app.ActivityOptions                                  |               34 |
     | android.view.LayoutInflater                                  |               31 |
     | android.app.Activity                                         |               28 |
     | android.content.SharedPreferences                            |               26 |
     | android.content.SharedPreferences.Editor                     |               26 |
     | android.text.SpannableStringBuilder                          |               23 |
     | android.view.ViewGroup.MarginLayoutParams                    |               21 |
     | ... (99 more items                                           |                  |
     ```

 Top referenced un-annotated members:

     ```
     | Member                                                       |      Usage Count |
     |--------------------------------------------------------------|-----------------:|
     | Parcel.readString()                                          |               62 |
     | Parcel.writeString(String)                                   |               62 |
     | TextView.setText(CharSequence)                               |               34 |
     | TransitionValues.values                                      |               28 |
     | View.getContext()                                            |               28 |
     | ViewPropertyAnimator.setDuration(long)                       |               26 |
     | ViewPropertyAnimator.setInterpolator(android.animation.Ti... |               26 |
     | LayoutInflater.inflate(int, android.view.ViewGroup, boole... |               23 |
     | Rect.left                                                    |               22 |
     | Rect.top                                                     |               22 |
     | Intent.Intent(android.content.Context, Class<?>)             |               21 |
     | Rect.bottom                                                  |               21 |
     | TransitionValues.view                                        |               21 |
     | VERSION.SDK_INT                                              |               18 |
     | Context.getResources()                                       |               18 |
     | EditText.getText()                                           |               18 |
     | ... (309 more items                                          |                  |
     ```

   From this it's clear that it would be useful to start annotating
   android.os.Parcel and android.view.View for example where there are
   unannotated APIs that are frequently used, at least by this app.

 * Built on top of a full, type-resolved AST. Doclava1 was integrated with
   javadoc, which meant that most of the source tree was opaque. Therefore, as
   just one example, the code which generated documentation for typedef constants
   had to require the constants to all share a single prefix it could look
   for. However, in metalava, annotation references are available at the AST
   level, so it can resolve references and map them back to the original field
   references and include those directly.

 * Support for extracting annotations. Metalava can also generate the external
   annotation files needed by Studio and lint in Gradle, which captures the
   typedefs (@IntDef and @StringDef classes) in the source code. Prior to this
   this was generated manually via the development/tools/extract code. This also
   merges in manually curated data; some of this is in the manual/ folder in this
   project.

 * Support for extracting API levels (api-versions.xml). This was generated by
   separate code (tools/base/misc/api-generator), invoked during the build. This
   functionality is now rolled into metalava, which has one very important
   attribute: metalava will use this information when recording API levels for
   API usage. (Prior to this, this was based on signature file parsing in
   doclava, which sometimes generated incorrect results. Metalava uses the
   android.jar files themselves to ensure that it computes the exact available
   SDK data for each API level.)

 * Misc other features. For example, if you use the @VisibleForTesting annotation
   from the support library, where you can express the intended visibility if the
   method had not required visibility for testing, then metalava will treat that
   method using the intended visibility instead when generating signature files
   and stubs.

 ## Architecture & Implementation

 Metalava is implemented on top of IntelliJ parsing APIs (PSI and UAST). However,
 these are hidden behind a "model": an abstraction layer which only exposes high
 level concepts like packages, classes and inner classes, methods, fields, and
 modifier lists (including annotations).

 This is done for multiple reasons:

 (1) It allows us to have multiple "back-ends": for example, metalava can read in
     a model not just from parsing source code, but from reading older SDK
     android.jar files (e.g. backed by bytecode) or reading previous signature
     files.  Reading in multiple versions of an API lets doclava perform
     "diffing", such as warning if an API is changing in an incompatible way. It
     can also generate signature files in the new format (including data that was
     missing in older signature files, such as annotation methods) without having
     to parse older source code which may no longer be easy to parse.

 (2) There's a lot of logic for deciding whether code found in the source tree
     should be included in the API. With the model approach we can build up an
     API and for example mark a subset of its methods as included. By having a
     separate hierarchy we can easily perform this work once and pass around our
     filtered model instead of passing around PsiClass and PsiMethod instances
     and having to keep the filtered data separately and remembering to always
     consult the filter, not the PSI elements directly.

 The basic API element class is "Item". (In doclava1 this was called a
 "DocInfo".)  There are several sub interfaces of Item: PackageItem, ClassItem,
 MemberItem, MethodItem, FieldItem, ParameterItem, etc. And then there are
 several implementation hierarchies: One is PSI based, where you point metalava
 to a source tree or a .jar file, and it constructs Items built on top of PSI:
 PsiPackageItem, PsiClassItem, PsiMethodItem, etc. Another is textual, based on
 signature files: TextPackageItem, TextClassItem, and so on.

 The "Codebase" class captures a complete API snapshot (including classes that
 are hidden, which is why it's called a "Codebase" rather than an "API").

 There are methods to load codebases - from source folders, from a .jar file,
 from a signature file. That's how API diffing is performed: you load two
 codebases (from whatever source you want, typically a previous API signature
 file and the current set of source folders), and then you "diff" the two.

 There are several key helpers that help with the implementation, detailed next.

 ### Visiting Items

 First, metalava provides an ItemVisitor. This lets you visit the API easily.
 For example, here's how you can visit every class:

     coebase.accept(object : ItemVisitor() {
         override fun visitClass(cls: ClassItem) {
             // code operating on the class here
         }
     })

 Similarly you can visit all items (regardless of type) by overriding
 `visitItem`, or to specifically visit methods, fields and so on overriding
 `visitPackage`, `visitClass`, `visitMethod`, etc.

 There is also an `ApiVisitor`. This is a subclass of the `ItemVisitor`, but
 which limits itself to visiting code elements that are part of the API.

 This is how for example the SignatureWriter and the StubWriter are both
 implemented: they simply extend `ApiVisitor`, which means they'll only export
 the API items in the codebase, and then in each relevant method they emit the
 signature or stub data:

     class SignatureWriter(
             private val writer: PrintWriter,
             private val generateDefaultConstructors: Boolean,
             private val filter: (Item) -> Boolean) : ApiVisitor(
             visitConstructorsAsMethods = false) {

     ....

     override fun visitConstructor(constructor: ConstructorItem) {
         writer.print("    ctor ")
         writeModifiers(constructor)
         writer.print(constructor.containingClass().fullName())
         writeParameterList(constructor)
         writeThrowsList(constructor)
         writer.print(";\n")
     }

     ....

 ### Visiting Types

 There is a `TypeVisitor` similar to `ItemVisitor` which you can use to visit all
 types in the codebase.

 When computing the API, all types that are included in the API should be
 included (e.g. if `List<Foo>` is part of the API then `Foo` must be too).  This
 is easy to do with the `TypeVisitor`.

 ### Diffing Codebases

 Another visitor which helps with implementation is the ComparisonVisitor:

     open class ComparisonVisitor {
         open fun compare(old: Item, new: Item) {}
         open fun added(item: Item) {}
         open fun removed(item: Item) {}

         open fun compare(old: PackageItem, new: PackageItem) { }
         open fun compare(old: ClassItem, new: ClassItem) { }
         open fun compare(old: MethodItem, new: MethodItem) { }
         open fun compare(old: FieldItem, new: FieldItem) { }
         open fun compare(old: ParameterItem, new: ParameterItem) { }

         open fun added(item: PackageItem) { }
         open fun added(item: ClassItem) { }
         open fun added(item: MethodItem) { }
         open fun added(item: FieldItem) { }
         open fun added(item: ParameterItem) { }

         open fun removed(item: PackageItem) { }
         open fun removed(item: ClassItem) { }
         open fun removed(item: MethodItem) { }
         open fun removed(item: FieldItem) { }
         open fun removed(item: ParameterItem) { }
     }

 This makes it easy to perform API comparison operations.

 For example, metalava has a feature to mark "newly annotated" nullness
 annotations as migrated. To do this, it just extends `ComparisonVisitor`,
 overrides the `compare(old: Item, new: Item)` method, and checks whether the old
 item has no nullness annotations and the new one does, and if so, also marks the
 new annotations as @Migrate.

 Similarly, the API Check can simply override

     open fun removed(item: Item) {
         reporter.report(error, item, "Removing ${Item.describe(item)} is not allowed")
     }

 to flag all API elements that have been removed as invalid (since you cannot
 remove API. (The real check is slightly more complicated; it looks into the
 hierarchy to see if there still is an inherited method with the same signature,
 in which case the deletion is allowed.))

 ### Documentation Generation

 As mentioned above, metalava generates documentation directly into the stubs
 files, which can then be processed by Dokka and Javadoc to generate the same
 docs as before.

 Doclava1 was integrated with javadoc directly, so the way it generated metadata
 docs (such as documenting permissions, ranges and typedefs from annotations) was
 to insert auxiliary tags (`@range`, `@permission`, etc) and then this would get
 converted into English docs later via `macros_override.cs`.

 This it not how metalava does it; it generates the English documentation
 directly. This was not just convenient for the implementation (since metalava
 does not use javadoc data structures to pass maps like the arguments for the
 typedef macro), but should also help Dokka -- and arguably the Kotlin code which
 generates the documentation is easier to reason about and to update when it's
 handling loop conditionals. (As a result I for example improved some of the
 grammar, e.g. when it's listing a number of possible constants the conjunction
 is usually "or", but if it's a flag, the sentence begins with "a combination of
 " and then the conjunction at the end should be "and").
	# Metalava

	Metalava is a metadata generator intended for JVM type projects. The main
	users of this tool are Android Platform and AndroidX libraries, however this
	tool also works on non-Android libraries.

	Metalava has many features related to API management. Some examples of the most
	commonly used ones are:

	* Allows extracting the API (into signature text files, into stub API files
	which in turn get compiled into android.jar, the Android SDK library) and
	more importantly to hide code intended to be implementation only, driven by
	javadoc comments like @hide, @doconly, @removed, etc, as well as various
	annotations.

	* Extracting source level annotations into external annotations file (such as
	the typedef annotations, which cannot be stored in the SDK as .class level
	annotations) to ship alongside the Android SDK and used by Android Lint.

	* Diffing versions of the API and determining whether a newer version is
	compatible with the older version. (See [COMPATIBILITY.md](COMPATIBILITY.md))

	## Building and running

	To download the code and any dependencies required for building, see [DOWNLOADING.md](DOWNLOADING.md)

	To build:

	$ cd tools/metalava
	$ ./gradlew

	It puts build artifacts in `../../out/metalava/`.

	To run the metalava executable:

	### Through Gradle

	To list all the options:

	$ ./gradlew run

	To run it with specific arguments:

	$ ./gradlew run --args="--api path/to/api.txt"

	### Through distribution artifact

	First build it with:

	$ ./gradlew installDist

	Then run it with:

	$ ../../out/metalava/install/metalava/bin/metalava
	_ _
	_ __ ___ ___\| \|_ __ _\| \| __ ___ ____ _
	\| '_ ` _ \ / _ \ __/ _` \| \|/ _` \ \ / / _` \|
	\| \| \| \| \| \| __/ \|\| (_\| \| \| (_\| \|\ V / (_\| \|
	\|_\| \|_\| \|_\|\___\|\__\__,_\|_\|\__,_\| \_/ \__,_\|

	metalava extracts metadata from source code to generate artifacts such as the
	signature files, the SDK stub files, external annotations etc.

	Usage: metalava <flags>

	Flags:

	--help This message.
	--quiet Only include vital output
	--verbose Include extra diagnostic output

	...

	(output truncated)

	## Features

	* Ability to read in an existing android.jar file instead of from source, which
	means we can regenerate signature files etc for older versions according to
	new formats (e.g. to fix past errors in doclava, such as annotation instance
	methods which were accidentally not included.)

	* Ability to merge in data (annotations etc) from external sources, such as
	IntelliJ external annotations data as well as signature files containing
	annotations. This isn't just merged at export time, it's merged at codebase
	load time such that it can be part of the API analysis.

	* Support for an updated signature file format (which is described in [FORMAT.md](FORMAT.md))

	* Address errors in the doclava1 format which for example was missing
	annotation class instance methods

	* Improve the signature format such that it for example labels enums "enum"
	instead of "abstract class extends java.lang.Enum", annotations as
	"@interface" instead of "abstract class extends java.lang.Annotation", sorts
	modifiers in the canonical modifier order, using "extends" instead of
	"implements" for the superclass of an interface, and many other similar
	tweaks outlined in the `Compatibility` class. (Metalava also allows (and
	ignores) block comments in the signature files.)

	* Add support for writing (and reading) annotations into the signature
	files. This is vital now that some of these annotations become part of the
	API contract (in particular nullness contracts, as well as parameter names
	and default values.)

	* Support for a "compact" nullness format -- one based on Kotlin's
	syntax. Since the goal is to have all API elements explicitly state
	their nullness contract, the signature files would very quickly become
	bloated with @NonNull and @Nullable annotations everywhere. So instead, the
	signature format now uses a suffix of `?` for nullable, `!` for not yet
	annotated, and nothing for non-null.

	Instead of

	method public java.lang.Double convert0(java.lang.Float);
	method @Nullable public java.lang.Double convert1(@NonNull java.lang.Float);

	we have

	method public java.lang.Double! convert0(java.lang.Float!);
	method public java.lang.Double? convert1(java.lang.Float);

	* Other compactness improvements: Skip packages in some cases both for export
	and reinsert during import. Specifically, drop "java.lang." from package
	names such that you have

	method public void onUpdate(int, String);

	instead of

	method public void onUpdate(int, java.lang.String);

	Similarly, annotations (the ones considered part of the API; unknown
	annotations are not included in signature files) use just the simple name
	instead of the full package name, e.g. `@UiThread` instead of
	`@android.annotation.UiThread`.

	* Misc documentation handling; for example, it attempts to fix sentences that
	javadoc will mistreat, such as sentences that "end" with "e.g. ". It also
	looks for various common typos and fixes those; here's a sample error
	message running metalava on master: Enhancing docs:

	frameworks/base/core/java/android/content/res/AssetManager.java:166: error: Replaced Kitkat with KitKat in documentation for Method android.content.res.AssetManager.getLocales() [Typo]
	frameworks/base/core/java/android/print/PrinterCapabilitiesInfo.java:122: error: Replaced Kitkat with KitKat in documentation for Method android.print.PrinterCapabilitiesInfo.Builder.setColorModes(int, int) [Typo]

	* Built-in support for injecting new annotations for use by the Kotlin compiler,
	not just nullness annotations found in the source code and annotations merged
	in from external sources, but also inferring whether nullness annotations have
	recently changed and if so marking them as @Migrate (which lets the Kotlin
	compiler treat errors in the user code as warnings instead of errors.)

	* Support for generating documentation into the stubs files (so we can run
	javadoc or [Dokka](https://github.com/Kotlin/dokka) on the stubs files instead
	of the source code). This means that the documentation tool itself does not
	need to be able to figure out which parts of the source code is included in
	the API and which one is implementation; it is simply handed the filtered API
	stub sources that include documentation.

	* Support for parsing Kotlin files. API files can now be implemented in Kotlin
	as well and metalava will parse and extract API information from them just as
	is done for Java files.

	* Like doclava1, metalava can diff two APIs and warn about API compatibility
	problems such as removing API elements. Metalava adds new warnings around
	nullness, such as attempting to change a nullness contract incompatibly
	(e.g. you can change a parameter from non null to nullable for final classes,
	but not versa). It also lets you diff directly on a source tree; it does not
	require you to create two signature files to diff.

	* Consistent stubs: In doclava1, the code which iterated over the API and
	generated the signature files and generated the stubs had diverged, so there
	was some inconsistency. In metalava the stub files contain exactly the
	same signatures as in the signature files.

	(This turned out to be incredibly important; this revealed for example that
	StringBuilder.setLength(int) was missing from the API signatures since it is a
	public method inherited from a package protected super class, which the API
	extraction code in doclava1 missed, but accidentally included in the SDK
	anyway since it packages package private classes. Metalava strictly applies
	the exact same API as is listed in the signature files, and once this was
	hooked up to the build it immediately became apparent that it was missing
	important methods that should really be part of the API.)

	* API Lint: Metalava can optionally (with --api-lint) run a series of additional
	checks on the public API in the codebase and flag issues that are discouraged
	or forbidden by the Android API Council; there are currently around 80 checks.
	Some of these take advantage of looking at the source code which wasn't
	possible with the signature-file based Python version; for example, it looks
	inside method bodies to see if you're synchronizing on this or the current
	class, which is forbidden.

	* Baselines: Metalava can report all of its issues into a "baseline" file, which
	records the current set of issues. From that point forward, when metalava
	finds a problem, it will only be reported if it is not already in the
	baseline. This lets you enforce new issues going forward without having to
	fix all existing violations. Periodically, as older issues are fixed, you can
	regenerate the baseline. For issues with some false positives, such as API
	Lint, being able to check in the set of accepted or verified false positives
	is quite important.

	* Metalava can generate reports about nullness annotation coverage (which helps
	target efforts since we plan to annotate the entire API). First, it can
	generate a raw count:

	Nullness Annotation Coverage Statistics:
	1279 out of 46900 methods were annotated (2%)
	2 out of 21683 fields were annotated (0%)
	2770 out of 47492 parameters were annotated (5%)

	More importantly, you can also point it to some existing compiled applications
	(.class or .jar files) and it will then measure the annotation coverage of the
	APIs used by those applications. This lets us target the most important APIs
	that are currently used by a corpus of apps and target our annotation efforts
	in a targeted way. For example, running the analysis on the current version of
	framework, and pointing it to the
	[Plaid](https://github.com/nickbutcher/plaid) app's compiled output with

	... --annotation-coverage-of ~/plaid/app/build/intermediates/classes/debug

	This produces the following output:

	324 methods and fields were missing nullness annotations out of 650 total
	API references. API nullness coverage is 50%

	```
	\| Qualified Class Name \| Usage Count \|
	\|--------------------------------------------------------------\|-----------------:\|
	\| android.os.Parcel \| 146 \|
	\| android.view.View \| 119 \|
	\| android.view.ViewPropertyAnimator \| 114 \|
	\| android.content.Intent \| 104 \|
	\| android.graphics.Rect \| 79 \|
	\| android.content.Context \| 61 \|
	\| android.widget.TextView \| 53 \|
	\| android.transition.TransitionValues \| 49 \|
	\| android.animation.Animator \| 34 \|
	\| android.app.ActivityOptions \| 34 \|
	\| android.view.LayoutInflater \| 31 \|
	\| android.app.Activity \| 28 \|
	\| android.content.SharedPreferences \| 26 \|
	\| android.content.SharedPreferences.Editor \| 26 \|
	\| android.text.SpannableStringBuilder \| 23 \|
	\| android.view.ViewGroup.MarginLayoutParams \| 21 \|
	\| ... (99 more items \| \|
	```

	Top referenced un-annotated members:

	```
	\| Member \| Usage Count \|
	\|--------------------------------------------------------------\|-----------------:\|
	\| Parcel.readString() \| 62 \|
	\| Parcel.writeString(String) \| 62 \|
	\| TextView.setText(CharSequence) \| 34 \|
	\| TransitionValues.values \| 28 \|
	\| View.getContext() \| 28 \|
	\| ViewPropertyAnimator.setDuration(long) \| 26 \|
	\| ViewPropertyAnimator.setInterpolator(android.animation.Ti... \| 26 \|
	\| LayoutInflater.inflate(int, android.view.ViewGroup, boole... \| 23 \|
	\| Rect.left \| 22 \|
	\| Rect.top \| 22 \|
	\| Intent.Intent(android.content.Context, Class<?>) \| 21 \|
	\| Rect.bottom \| 21 \|
	\| TransitionValues.view \| 21 \|
	\| VERSION.SDK_INT \| 18 \|
	\| Context.getResources() \| 18 \|
	\| EditText.getText() \| 18 \|
	\| ... (309 more items \| \|
	```

	From this it's clear that it would be useful to start annotating
	android.os.Parcel and android.view.View for example where there are
	unannotated APIs that are frequently used, at least by this app.

	* Built on top of a full, type-resolved AST. Doclava1 was integrated with
	javadoc, which meant that most of the source tree was opaque. Therefore, as
	just one example, the code which generated documentation for typedef constants
	had to require the constants to all share a single prefix it could look
	for. However, in metalava, annotation references are available at the AST
	level, so it can resolve references and map them back to the original field
	references and include those directly.

	* Support for extracting annotations. Metalava can also generate the external
	annotation files needed by Studio and lint in Gradle, which captures the
	typedefs (@IntDef and @StringDef classes) in the source code. Prior to this
	this was generated manually via the development/tools/extract code. This also
	merges in manually curated data; some of this is in the manual/ folder in this
	project.

	* Support for extracting API levels (api-versions.xml). This was generated by
	separate code (tools/base/misc/api-generator), invoked during the build. This
	functionality is now rolled into metalava, which has one very important
	attribute: metalava will use this information when recording API levels for
	API usage. (Prior to this, this was based on signature file parsing in
	doclava, which sometimes generated incorrect results. Metalava uses the
	android.jar files themselves to ensure that it computes the exact available
	SDK data for each API level.)

	* Misc other features. For example, if you use the @VisibleForTesting annotation
	from the support library, where you can express the intended visibility if the
	method had not required visibility for testing, then metalava will treat that
	method using the intended visibility instead when generating signature files
	and stubs.

	## Architecture & Implementation

	Metalava is implemented on top of IntelliJ parsing APIs (PSI and UAST). However,
	these are hidden behind a "model": an abstraction layer which only exposes high
	level concepts like packages, classes and inner classes, methods, fields, and
	modifier lists (including annotations).

	This is done for multiple reasons:

	(1) It allows us to have multiple "back-ends": for example, metalava can read in
	a model not just from parsing source code, but from reading older SDK
	android.jar files (e.g. backed by bytecode) or reading previous signature
	files. Reading in multiple versions of an API lets doclava perform
	"diffing", such as warning if an API is changing in an incompatible way. It
	can also generate signature files in the new format (including data that was
	missing in older signature files, such as annotation methods) without having
	to parse older source code which may no longer be easy to parse.

	(2) There's a lot of logic for deciding whether code found in the source tree
	should be included in the API. With the model approach we can build up an
	API and for example mark a subset of its methods as included. By having a
	separate hierarchy we can easily perform this work once and pass around our
	filtered model instead of passing around PsiClass and PsiMethod instances
	and having to keep the filtered data separately and remembering to always
	consult the filter, not the PSI elements directly.

	The basic API element class is "Item". (In doclava1 this was called a
	"DocInfo".) There are several sub interfaces of Item: PackageItem, ClassItem,
	MemberItem, MethodItem, FieldItem, ParameterItem, etc. And then there are
	several implementation hierarchies: One is PSI based, where you point metalava
	to a source tree or a .jar file, and it constructs Items built on top of PSI:
	PsiPackageItem, PsiClassItem, PsiMethodItem, etc. Another is textual, based on
	signature files: TextPackageItem, TextClassItem, and so on.

	The "Codebase" class captures a complete API snapshot (including classes that
	are hidden, which is why it's called a "Codebase" rather than an "API").

	There are methods to load codebases - from source folders, from a .jar file,
	from a signature file. That's how API diffing is performed: you load two
	codebases (from whatever source you want, typically a previous API signature
	file and the current set of source folders), and then you "diff" the two.

	There are several key helpers that help with the implementation, detailed next.

	### Visiting Items

	First, metalava provides an ItemVisitor. This lets you visit the API easily.
	For example, here's how you can visit every class:

	coebase.accept(object : ItemVisitor() {
	override fun visitClass(cls: ClassItem) {
	// code operating on the class here
	}
	})

	Similarly you can visit all items (regardless of type) by overriding
	`visitItem`, or to specifically visit methods, fields and so on overriding
	`visitPackage`, `visitClass`, `visitMethod`, etc.

	There is also an `ApiVisitor`. This is a subclass of the `ItemVisitor`, but
	which limits itself to visiting code elements that are part of the API.

	This is how for example the SignatureWriter and the StubWriter are both
	implemented: they simply extend `ApiVisitor`, which means they'll only export
	the API items in the codebase, and then in each relevant method they emit the
	signature or stub data:

	class SignatureWriter(
	private val writer: PrintWriter,
	private val generateDefaultConstructors: Boolean,
	private val filter: (Item) -> Boolean) : ApiVisitor(
	visitConstructorsAsMethods = false) {

	....

	override fun visitConstructor(constructor: ConstructorItem) {
	writer.print(" ctor ")
	writeModifiers(constructor)
	writer.print(constructor.containingClass().fullName())
	writeParameterList(constructor)
	writeThrowsList(constructor)
	writer.print(";\n")
	}

	....

	### Visiting Types

	There is a `TypeVisitor` similar to `ItemVisitor` which you can use to visit all
	types in the codebase.

	When computing the API, all types that are included in the API should be
	included (e.g. if `List<Foo>` is part of the API then `Foo` must be too). This
	is easy to do with the `TypeVisitor`.

	### Diffing Codebases

	Another visitor which helps with implementation is the ComparisonVisitor:

	open class ComparisonVisitor {
	open fun compare(old: Item, new: Item) {}
	open fun added(item: Item) {}
	open fun removed(item: Item) {}

	open fun compare(old: PackageItem, new: PackageItem) { }
	open fun compare(old: ClassItem, new: ClassItem) { }
	open fun compare(old: MethodItem, new: MethodItem) { }
	open fun compare(old: FieldItem, new: FieldItem) { }
	open fun compare(old: ParameterItem, new: ParameterItem) { }

	open fun added(item: PackageItem) { }
	open fun added(item: ClassItem) { }
	open fun added(item: MethodItem) { }
	open fun added(item: FieldItem) { }
	open fun added(item: ParameterItem) { }

	open fun removed(item: PackageItem) { }
	open fun removed(item: ClassItem) { }
	open fun removed(item: MethodItem) { }
	open fun removed(item: FieldItem) { }
	open fun removed(item: ParameterItem) { }
	}

	This makes it easy to perform API comparison operations.

	For example, metalava has a feature to mark "newly annotated" nullness
	annotations as migrated. To do this, it just extends `ComparisonVisitor`,
	overrides the `compare(old: Item, new: Item)` method, and checks whether the old
	item has no nullness annotations and the new one does, and if so, also marks the
	new annotations as @Migrate.

	Similarly, the API Check can simply override

	open fun removed(item: Item) {
	reporter.report(error, item, "Removing ${Item.describe(item)} is not allowed")
	}

	to flag all API elements that have been removed as invalid (since you cannot
	remove API. (The real check is slightly more complicated; it looks into the
	hierarchy to see if there still is an inherited method with the same signature,
	in which case the deletion is allowed.))

	### Documentation Generation

	As mentioned above, metalava generates documentation directly into the stubs
	files, which can then be processed by Dokka and Javadoc to generate the same
	docs as before.

	Doclava1 was integrated with javadoc directly, so the way it generated metadata
	docs (such as documenting permissions, ranges and typedefs from annotations) was
	to insert auxiliary tags (`@range`, `@permission`, etc) and then this would get
	converted into English docs later via `macros_override.cs`.

	This it not how metalava does it; it generates the English documentation
	directly. This was not just convenient for the implementation (since metalava
	does not use javadoc data structures to pass maps like the arguments for the
	typedef macro), but should also help Dokka -- and arguably the Kotlin code which
	generates the documentation is easier to reason about and to update when it's
	handling loop conditionals. (As a result I for example improved some of the
	grammar, e.g. when it's listing a number of possible constants the conjunction
	is usually "or", but if it's a flag, the sentence begins with "a combination of
	" and then the conjunction at the end should be "and").