1# Metalava 2 3(Also known as "doclava2", but deliberately not named doclava2 since crucially 4it does not generate docs; it's intended only for **meta**data extraction and 5generation.) 6 7Metalava is a metadata generator intended for the Android source tree, used for 8a number of purposes: 9 10* Allow extracting the API (into signature text files, into stub API files 11 (which in turn get compiled into android.jar, the Android SDK library) and 12 more importantly to hide code intended to be implementation only, driven by 13 javadoc comments like @hide, @$doconly, @removed, etc, as well as various 14 annotations. 15 16* Extracting source level annotations into external annotations file (such as 17 the typedef annotations, which cannot be stored in the SDK as .class level 18 annotations). 19 20* Diffing versions of the API and determining whether a newer version is 21 compatible with the older version. 22 23## Building and running 24 25To build: 26 27 $ ./gradlew 28 29This builds a binary distribution in `../../out/host/common/install/metalava/bin/metalava`. 30 31To run metalava: 32 33 $ ../../out/host/common/install/metalava/bin/metalava 34 _ _ 35 _ __ ___ ___| |_ __ _| | __ ___ ____ _ 36 | '_ ` _ \ / _ \ __/ _` | |/ _` \ \ / / _` | 37 | | | | | | __/ || (_| | | (_| |\ V / (_| | 38 |_| |_| |_|\___|\__\__,_|_|\__,_| \_/ \__,_| 39 40 metalava extracts metadata from source code to generate artifacts such as the 41 signature files, the SDK stub files, external annotations etc. 42 43 Usage: metalava <flags> 44 45 Flags: 46 47 --help This message. 48 --quiet Only include vital output 49 --verbose Include extra diagnostic output 50 51 ... 52 53(*output truncated*) 54 55Metalava has a new command line syntax, but it also understands the doclava1 56flags and translates them on the fly. Flags that are ignored are listed on the 57command line. If metalava is dropped into an Android framework build for 58example, you'll see something like this (unless running with --quiet) : 59 60 metalava: Ignoring javadoc-related doclava1 flag -J-Xmx1600m 61 metalava: Ignoring javadoc-related doclava1 flag -J-XX:-OmitStackTraceInFastThrow 62 metalava: Ignoring javadoc-related doclava1 flag -XDignore.symbol.file 63 metalava: Ignoring javadoc-related doclava1 flag -doclet 64 metalava: Ignoring javadoc-related doclava1 flag -docletpath 65 metalava: Ignoring javadoc-related doclava1 flag -templatedir 66 metalava: Ignoring javadoc-related doclava1 flag -htmldir 67 ... 68 69## Features 70 71* Compatibility with doclava1: in compat mode, metalava spits out the same 72 signature files for the framework as doclava1. 73 74* Ability to read in an existing android.jar file instead of from source, which 75 means we can regenerate signature files etc for older versions according to 76 new formats (e.g. to fix past errors in doclava, such as annotation instance 77 methods which were accidentally not included.) 78 79* Ability to merge in data (annotations etc) from external sources, such as 80 IntelliJ external annotations data as well as signature files containing 81 annotations. This isn't just merged at export time, it's merged at codebase 82 load time such that it can be part of the API analysis. 83 84* Support for an updated signature file format (which is described in FORMAT.md) 85 86 * Address errors in the doclava1 format which for example was missing 87 annotation class instance methods 88 89 * Improve the signature format such that it for example labels enums "enum" 90 instead of "abstract class extends java.lang.Enum", annotations as 91 "@interface" instead of "abstract class extends java.lang.Annotation", sorts 92 modifiers in the canonical modifier order, using "extends" instead of 93 "implements" for the superclass of an interface, and many other similar 94 tweaks outlined in the `Compatibility` class. (Metalava also allows (and 95 ignores) block comments in the signature files.) 96 97 * Add support for writing (and reading) annotations into the signature 98 files. This is vital now that some of these annotations become part of the 99 API contract (in particular nullness contracts, as well as parameter names 100 and default values.) 101 102 * Support for a "compact" nullness format -- one based on Kotlin's 103 syntax. Since the goal is to have **all** API elements explicitly state 104 their nullness contract, the signature files would very quickly become 105 bloated with @NonNull and @Nullable annotations everywhere. So instead, the 106 signature format now uses a suffix of `?` for nullable, `!` for not yet 107 annotated, and nothing for non-null. 108 109 Instead of 110 111 method public java.lang.Double convert0(java.lang.Float); 112 method @Nullable public java.lang.Double convert1(@NonNull java.lang.Float); 113 114 we have 115 116 method public java.lang.Double! convert0(java.lang.Float!); 117 method public java.lang.Double? convert1(java.lang.Float); 118 119 * Other compactness improvements: Skip packages in some cases both for export 120 and reinsert during import. Specifically, drop "java.lang." from package 121 names such that you have 122 123 method public void onUpdate(int, String); 124 125 instead of 126 127 method public void onUpdate(int, java.lang.String); 128 129 Similarly, annotations (the ones considered part of the API; unknown 130 annotations are not included in signature files) use just the simple name 131 instead of the full package name, e.g. `@UiThread` instead of 132 `@android.annotation.UiThread`. 133 134 * Misc documentation handling; for example, it attempts to fix sentences that 135 javadoc will mistreat, such as sentences that "end" with "e.g. ". It also 136 looks for various common typos and fixes those; here's a sample error 137 message running metalava on master: Enhancing docs: 138 139 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] 140 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] 141 142* Built-in support for injecting new annotations for use by the Kotlin compiler, 143 not just nullness annotations found in the source code and annotations merged 144 in from external sources, but also inferring whether nullness annotations have 145 recently changed and if so marking them as @Migrate (which lets the Kotlin 146 compiler treat errors in the user code as warnings instead of errors.) 147 148* Support for generating documentation into the stubs files (so we can run 149 javadoc or [Dokka](https://github.com/Kotlin/dokka) on the stubs files instead 150 of the source code). This means that the documentation tool itself does not 151 need to be able to figure out which parts of the source code is included in 152 the API and which one is implementation; it is simply handed the filtered API 153 stub sources that include documentation. 154 155* Support for parsing Kotlin files. API files can now be implemented in Kotlin 156 as well and metalava will parse and extract API information from them just as 157 is done for Java files. 158 159* Like doclava1, metalava can diff two APIs and warn about API compatibility 160 problems such as removing API elements. Metalava adds new warnings around 161 nullness, such as attempting to change a nullness contract incompatibly 162 (e.g. you can change a parameter from non null to nullable for final classes, 163 but not versa). It also lets you diff directly on a source tree; it does not 164 require you to create two signature files to diff. 165 166* Consistent stubs: In doclava1, the code which iterated over the API and 167 generated the signature files and generated the stubs had diverged, so there 168 was some inconsistency. In metalava the stub files contain **exactly** the 169 same signatures as in the signature files. 170 171 (This turned out to be incredibly important; this revealed for example that 172 StringBuilder.setLength(int) was missing from the API signatures since it is a 173 public method inherited from a package protected super class, which the API 174 extraction code in doclava1 missed, but accidentally included in the SDK 175 anyway since it packages package private classes. Metalava strictly applies 176 the exact same API as is listed in the signature files, and once this was 177 hooked up to the build it immediately became apparent that it was missing 178 important methods that should really be part of the API.) 179 180* API Lint: Metalava can optionally (with --api-lint) run a series of additional 181 checks on the public API in the codebase and flag issues that are discouraged 182 or forbidden by the Android API Council; there are currently around 80 checks. 183 Some of these take advantage of looking at the source code which wasn't 184 possible with the signature-file based Python version; for example, it looks 185 inside method bodies to see if you're synchronizing on this or the current 186 class, which is forbidden. 187 188* Baselines: Metalava can report all of its issues into a "baseline" file, which 189 records the current set of issues. From that point forward, when metalava 190 finds a problem, it will only be reported if it is not already in the 191 baseline. This lets you enforce new issues going forward without having to 192 fix all existing violations. Periodically, as older issues are fixed, you can 193 regenerate the baseline. For issues with some false positives, such as API 194 Lint, being able to check in the set of accepted or verified false positives 195 is quite important. 196 197* Metalava can generate reports about nullness annotation coverage (which helps 198 target efforts since we plan to annotate the entire API). First, it can 199 generate a raw count: 200 201 Nullness Annotation Coverage Statistics: 202 1279 out of 46900 methods were annotated (2%) 203 2 out of 21683 fields were annotated (0%) 204 2770 out of 47492 parameters were annotated (5%) 205 206 More importantly, you can also point it to some existing compiled applications 207 (.class or .jar files) and it will then measure the annotation coverage of the 208 APIs used by those applications. This lets us target the most important APIs 209 that are currently used by a corpus of apps and target our annotation efforts 210 in a targeted way. For example, running the analysis on the current version of 211 framework, and pointing it to the 212 [Plaid](https://github.com/nickbutcher/plaid) app's compiled output with 213 214 ... --annotation-coverage-of ~/plaid/app/build/intermediates/classes/debug 215 216 This produces the following output: 217 218 324 methods and fields were missing nullness annotations out of 650 total 219 API references. API nullness coverage is 50% 220 221 ``` 222 | Qualified Class Name | Usage Count | 223 |--------------------------------------------------------------|-----------------:| 224 | android.os.Parcel | 146 | 225 | android.view.View | 119 | 226 | android.view.ViewPropertyAnimator | 114 | 227 | android.content.Intent | 104 | 228 | android.graphics.Rect | 79 | 229 | android.content.Context | 61 | 230 | android.widget.TextView | 53 | 231 | android.transition.TransitionValues | 49 | 232 | android.animation.Animator | 34 | 233 | android.app.ActivityOptions | 34 | 234 | android.view.LayoutInflater | 31 | 235 | android.app.Activity | 28 | 236 | android.content.SharedPreferences | 26 | 237 | android.content.SharedPreferences.Editor | 26 | 238 | android.text.SpannableStringBuilder | 23 | 239 | android.view.ViewGroup.MarginLayoutParams | 21 | 240 | ... (99 more items | | 241 ``` 242 243Top referenced un-annotated members: 244 245 ``` 246 | Member | Usage Count | 247 |--------------------------------------------------------------|-----------------:| 248 | Parcel.readString() | 62 | 249 | Parcel.writeString(String) | 62 | 250 | TextView.setText(CharSequence) | 34 | 251 | TransitionValues.values | 28 | 252 | View.getContext() | 28 | 253 | ViewPropertyAnimator.setDuration(long) | 26 | 254 | ViewPropertyAnimator.setInterpolator(android.animation.Ti... | 26 | 255 | LayoutInflater.inflate(int, android.view.ViewGroup, boole... | 23 | 256 | Rect.left | 22 | 257 | Rect.top | 22 | 258 | Intent.Intent(android.content.Context, Class<?>) | 21 | 259 | Rect.bottom | 21 | 260 | TransitionValues.view | 21 | 261 | VERSION.SDK_INT | 18 | 262 | Context.getResources() | 18 | 263 | EditText.getText() | 18 | 264 | ... (309 more items | | 265 ``` 266 267 From this it's clear that it would be useful to start annotating 268 android.os.Parcel and android.view.View for example where there are 269 unannotated APIs that are frequently used, at least by this app. 270 271* Built on top of a full, type-resolved AST. Doclava1 was integrated with 272 javadoc, which meant that most of the source tree was opaque. Therefore, as 273 just one example, the code which generated documentation for typedef constants 274 had to require the constants to all share a single prefix it could look 275 for. However, in metalava, annotation references are available at the AST 276 level, so it can resolve references and map them back to the original field 277 references and include those directly. 278 279* Support for extracting annotations. Metalava can also generate the external 280 annotation files needed by Studio and lint in Gradle, which captures the 281 typedefs (@IntDef and @StringDef classes) in the source code. Prior to this 282 this was generated manually via the development/tools/extract code. This also 283 merges in manually curated data; some of this is in the manual/ folder in this 284 project. 285 286* Support for extracting API levels (api-versions.xml). This was generated by 287 separate code (tools/base/misc/api-generator), invoked during the build. This 288 functionality is now rolled into metalava, which has one very important 289 attribute: metalava will use this information when recording API levels for 290 API usage. (Prior to this, this was based on signature file parsing in 291 doclava, which sometimes generated incorrect results. Metalava uses the 292 android.jar files themselves to ensure that it computes the exact available 293 SDK data for each API level.) 294 295* Misc other features. For example, if you use the @VisibleForTesting annotation 296 from the support library, where you can express the intended visibility if the 297 method had not required visibility for testing, then metalava will treat that 298 method using the intended visibility instead when generating signature files 299 and stubs. 300 301## Architecture & Implementation 302 303Metalava is implemented on top of IntelliJ parsing APIs (PSI and UAST). However, 304these are hidden behind a "model": an abstraction layer which only exposes high 305level concepts like packages, classes and inner classes, methods, fields, and 306modifier lists (including annotations). 307 308This is done for multiple reasons: 309 310(1) It allows us to have multiple "back-ends": for example, metalava can read in 311 a model not just from parsing source code, but from reading older SDK 312 android.jar files (e.g. backed by bytecode) or reading previous signature 313 files. Reading in multiple versions of an API lets doclava perform 314 "diffing", such as warning if an API is changing in an incompatible way. It 315 can also generate signature files in the new format (including data that was 316 missing in older signature files, such as annotation methods) without having 317 to parse older source code which may no longer be easy to parse. 318 319(2) There's a lot of logic for deciding whether code found in the source tree 320 should be included in the API. With the model approach we can build up an 321 API and for example mark a subset of its methods as included. By having a 322 separate hierarchy we can easily perform this work once and pass around our 323 filtered model instead of passing around PsiClass and PsiMethod instances 324 and having to keep the filtered data separately and remembering to always 325 consult the filter, not the PSI elements directly. 326 327The basic API element class is "Item". (In doclava1 this was called a 328"DocInfo".) There are several sub interfaces of Item: PackageItem, ClassItem, 329MemberItem, MethodItem, FieldItem, ParameterItem, etc. And then there are 330several implementation hierarchies: One is PSI based, where you point metalava 331to a source tree or a .jar file, and it constructs Items built on top of PSI: 332PsiPackageItem, PsiClassItem, PsiMethodItem, etc. Another is textual, based on 333signature files: TextPackageItem, TextClassItem, and so on. 334 335The "Codebase" class captures a complete API snapshot (including classes that 336are hidden, which is why it's called a "Codebase" rather than an "API"). 337 338There are methods to load codebases - from source folders, from a .jar file, 339from a signature file. That's how API diffing is performed: you load two 340codebases (from whatever source you want, typically a previous API signature 341file and the current set of source folders), and then you "diff" the two. 342 343There are several key helpers that help with the implementation, detailed next. 344 345### Visiting Items 346 347First, metalava provides an ItemVisitor. This lets you visit the API easily. 348For example, here's how you can visit every class: 349 350 coebase.accept(object : ItemVisitor() { 351 override fun visitClass(cls: ClassItem) { 352 // code operating on the class here 353 } 354 }) 355 356Similarly you can visit all items (regardless of type) by overriding 357`visitItem`, or to specifically visit methods, fields and so on overriding 358`visitPackage`, `visitClass`, `visitMethod`, etc. 359 360There is also an `ApiVisitor`. This is a subclass of the `ItemVisitor`, but 361which limits itself to visiting code elements that are part of the API. 362 363This is how for example the SignatureWriter and the StubWriter are both 364implemented: they simply extend `ApiVisitor`, which means they'll only export 365the API items in the codebase, and then in each relevant method they emit the 366signature or stub data: 367 368 class SignatureWriter( 369 private val writer: PrintWriter, 370 private val generateDefaultConstructors: Boolean, 371 private val filter: (Item) -> Boolean) : ApiVisitor( 372 visitConstructorsAsMethods = false) { 373 374 .... 375 376 override fun visitConstructor(constructor: ConstructorItem) { 377 writer.print(" ctor ") 378 writeModifiers(constructor) 379 writer.print(constructor.containingClass().fullName()) 380 writeParameterList(constructor) 381 writeThrowsList(constructor) 382 writer.print(";\n") 383 } 384 385 .... 386 387### Visiting Types 388 389There is a `TypeVisitor` similar to `ItemVisitor` which you can use to visit all 390types in the codebase. 391 392When computing the API, all types that are included in the API should be 393included (e.g. if `List<Foo>` is part of the API then `Foo` must be too). This 394is easy to do with the `TypeVisitor`. 395 396### Diffing Codebases 397 398Another visitor which helps with implementation is the ComparisonVisitor: 399 400 open class ComparisonVisitor { 401 open fun compare(old: Item, new: Item) {} 402 open fun added(item: Item) {} 403 open fun removed(item: Item) {} 404 405 open fun compare(old: PackageItem, new: PackageItem) { } 406 open fun compare(old: ClassItem, new: ClassItem) { } 407 open fun compare(old: MethodItem, new: MethodItem) { } 408 open fun compare(old: FieldItem, new: FieldItem) { } 409 open fun compare(old: ParameterItem, new: ParameterItem) { } 410 411 open fun added(item: PackageItem) { } 412 open fun added(item: ClassItem) { } 413 open fun added(item: MethodItem) { } 414 open fun added(item: FieldItem) { } 415 open fun added(item: ParameterItem) { } 416 417 open fun removed(item: PackageItem) { } 418 open fun removed(item: ClassItem) { } 419 open fun removed(item: MethodItem) { } 420 open fun removed(item: FieldItem) { } 421 open fun removed(item: ParameterItem) { } 422 } 423 424This makes it easy to perform API comparison operations. 425 426For example, metalava has a feature to mark "newly annotated" nullness 427annotations as migrated. To do this, it just extends `ComparisonVisitor`, 428overrides the `compare(old: Item, new: Item)` method, and checks whether the old 429item has no nullness annotations and the new one does, and if so, also marks the 430new annotations as @Migrate. 431 432Similarly, the API Check can simply override 433 434 open fun removed(item: Item) { 435 reporter.report(error, item, "Removing ${Item.describe(item)} is not allowed") 436 } 437 438to flag all API elements that have been removed as invalid (since you cannot 439remove API. (The real check is slightly more complicated; it looks into the 440hierarchy to see if there still is an inherited method with the same signature, 441in which case the deletion is allowed.)) 442 443### Documentation Generation 444 445As mentioned above, metalava generates documentation directly into the stubs 446files, which can then be processed by Dokka and Javadoc to generate the same 447docs as before. 448 449Doclava1 was integrated with javadoc directly, so the way it generated metadata 450docs (such as documenting permissions, ranges and typedefs from annotations) was 451to insert auxiliary tags (`@range`, `@permission`, etc) and then this would get 452converted into English docs later via `macros_override.cs`. 453 454This it not how metalava does it; it generates the English documentation 455directly. This was not just convenient for the implementation (since metalava 456does not use javadoc data structures to pass maps like the arguments for the 457typedef macro), but should also help Dokka -- and arguably the Kotlin code which 458generates the documentation is easier to reason about and to update when it's 459handling loop conditionals. (As a result I for example improved some of the 460grammar, e.g. when it's listing a number of possible constants the conjunction 461is usually "or", but if it's a flag, the sentence begins with "a combination of 462" and then the conjunction at the end should be "and"). 463