# Chromium's Java Toolchain

This doc aims to describe the Chrome build process that takes a set of `.java`
files and turns them into a `classes.dex` file.

[TOC]

## Core GN Target Types

The following have `supports_android` and `requires_android` set to false by
default:
* `java_library()`: Compiles `.java` -> `.jar`
* `java_prebuilt()`:  Imports a prebuilt `.jar` file.

The following have `supports_android` and `requires_android` set to true. They
also have a default `jar_excluded_patterns` set (more on that later):
* `android_library()`
* `android_java_prebuilt()`

All target names must end with "_java" so that the build system can distinguish
them from non-java targets (or [other variations](https://cs.chromium.org/chromium/src/build/config/android/internal_rules.gni?rcl=ec2c17d7b4e424e060c3c7972842af87343526a1&l=20)).

## From Source to Final Dex

### Step 1a: Compile with javac

This step is the only step that does not apply to prebuilt targets.

* All `.java` files in a target are compiled by `javac` into `.class` files.
  * This includes `.java` files that live within `.srcjar` files, referenced
    through `srcjar_deps`.
* The `classpath` used when compiling a target is comprised of `.jar` files of
  its deps.
  * When deps are library targets, the Step 1 `.jar` file is used.
  * When deps are prebuilt targets, the original `.jar` file is used.
  * All `.jar` processing done in subsequent steps does not impact compilation
    classpath.
* `.class` files are zipped into an output `.jar` file.
* There is **no support** for incremental compilation at this level.
  * If one source file changes within a library, then the entire library is
    recompiled.
  * Prefer smaller targets to avoid slow compiles.

### Step 1b: Compile with ErrorProne

This step can be disabled via GN arg: `use_errorprone_java_compiler = false`

* Concurrently with step 1a: [ErrorProne] compiles java files and checks for bug
  patterns, including some [custom to Chromium][ep_plugins].
* ErrorProne used to replace step 1a, but was changed to a concurrent step after
  being identified as being slower.

[ErrorProne]: https://errorprone.info/
[ep_plugins]: /tools/android/errorprone_plugin/

### Step 2: Creating an .interface.jar

This step happens in parallel with subsequent steps.

* `//third_party/ijar` converts the `.jar` into an `.interface.jar`, which is a
  copy of the input with all non-public symbols and function bodies removed.
* Dependant targets use `.interface.jar` files to skip having to be rebuilt
  when only private implementation details change.
  * To accomplish this behavior, library targets list only their
    `.interface.jar` as outputs. Ninja's `restat=1` feature then causes
    dependent targets to be rebuilt only when the `.interface.jar` changes.
    Final dex targets are always rebuilt because they depend on the
    non-`.interface.jar` through a `depfile`.

### Step 3: Bytecode Processing

* `//build/android/bytecode` runs on the compiled `.jar` in order to:
  * Enable Java assertions (when dcheck is enabled).
  * Assert that libraries have properly declared `deps`.

### Step 4: Desugaring

This step happens only when targets have `supports_android = true`.

* `//third_party/bazel/desugar` converts certain Java 8 constructs, such as
  lambdas and default interface methods, into constructs that are compatible
  with Java 7.

### Step 5: Filtering

This step happens only when targets that have `jar_excluded_patterns` or
`jar_included_patterns` set (e.g. all `android_` targets).

* Remove `.class` files that match the filters from the `.jar`. These `.class`
  files are generally those that are re-created with different implementations
  further on in the build process.
  * E.g.: `R.class` files - a part of [Android Resources].
  * E.g.: `GEN_JNI.class` - a part of our [JNI] glue.
  * E.g.: `AppHooksImpl.class` - how `chrome_java` wires up different
    implementations for [non-public builds][apphooks].

[JNI]: /base/android/jni_generator/README.md
[Android Resources]: life_of_a_resource.md
[apphooks]: /chrome/android/java/src/org/chromium/chrome/browser/AppHooksImpl.java

### Step 6: Instrumentation

This step happens only when this GN arg is set: `use_jacoco_coverage = true`

* [Jacoco] adds instrumentation hooks to methods.

[Jacoco]: https://www.eclemma.org/jacoco/

### Step 7: Copy to lib.java

* The `.jar` is copied into `$root_build_dir/lib.java` (under target-specific
  subdirectories) so that it will be included by bot archive steps.
  * These `.jar` files are the ones used when running `java_binary` and
    `junit_binary` targets.

### Step 8: Per-Library Dexing

This step happens only when targets have `supports_android = true`.

* [d8] converts `.jar` files containing `.class` files into `.dex.jar` files
  containing `.dex` files.
* Dexing is incremental - it will reuse dex'ed classes from a previous build if
  the corresponding `.class` file is unchanged.
* These per-library `.dex.jar` files are used directly by [incremental install],
  and are inputs to the Apk step when `enable_proguard = false`.
  * Even when `is_java_debug = false`, many apk targets do not enable ProGuard
    (e.g. unit tests).

[d8]: https://developer.android.com/studio/command-line/d8
[incremental install]: /build/android/incremental_install/README.md

### Step 9: Apk / Bundle Module Compile

* Each `android_apk` and `android_bundle_module` template has a nested
  `java_library` target. The nested library includes final copies of files
  stripped out by prior filtering steps. These files include:
  * Final `R.java` files, created by `compile_resources.py`.
  * Final `GEN_JNI.java` for [JNI glue].
  * `BuildConfig.java` and `NativeLibraries.java` (//base dependencies).

[JNI glue]: /base/android/jni_generator/README.md

### Step 10: Final Dexing

This step is skipped when building using [Incremental Install].

When `is_java_debug = true`:
* [d8] merges all library `.dex.jar` files into a final `.dex.zip`.

When `is_java_debug = false`:
* [R8] performs whole-program optimization on all library `lib.java` `.jar`
  files and outputs a final `.dex.zip`.
  * For App Bundles, R8 creates a single `.dex.zip` with the code from all
    modules.

[Incremental Install]: /build/android/incremental_install/README.md
[R8]: https://r8.googlesource.com/r8

### Step 11: Bundle Module Dex Splitting

This step happens only when `is_java_debug = false`.

* [dexsplitter.py] splits the single `.dex.zip` into per-module `.dex.zip`
  files.

## Test APKs with apk_under_test

Test APKs are normal APKs that contain an `<instrumentation>` tag within their
`AndroidManifest.xml`. If this tag specifies an `android:targetPackage`
different from itself, then Android will add that package's `classes.dex` to the
test APK's Java classpath when run. In GN, you can enable this behavior using
the `apk_under_test` parameter on `instrumentation_test_apk` targets. Using it
is discouraged if APKs have `proguard_enabled=true`.

### Difference in Final Dex

When `enable_proguard=false`:
* Any library depended on by the test APK that is also depended on by the
  apk-under-test is excluded from the test APK's final dex step.

When `enable_proguard=true`:
* Test APKs cannot make use of the apk-under-test's dex because only symbols
  explicitly kept by `-keep` directives are guaranteed to exist after
  ProGuarding. As a work-around, test APKs include all of the apk-under-test's
  libraries directly in its own final dex such that the under-test apk's Java
  code is never used (because it is entirely shadowed by the test apk's dex).
  * We've found this configuration to be fragile, and are trying to [move away
    from it](https://bugs.chromium.org/p/chromium/issues/detail?id=890452).

### Difference in GEN_JNI.java
* Calling native methods using [JNI glue] requires that a `GEN_JNI.java` class
  be generated that contains all native methods for an APK. There cannot be
  conflicting `GEN_JNI` classes in both the test apk and the apk-under-test, so
  only the apk-under-test has one generated for it. As a result this,
  instrumentation test APKs that use apk-under-test cannot use native methods
  that aren't already part of the apk-under-test.

## How to Generate Java Source Code
There are two ways to go about generating source files: Annotation Processors
and custom build steps.

### Annotation Processors
* These are run by `javac` as part of the compile step.
* They **cannot** modify the source files that they apply to. They can only
  generate new sources.
* Use these when:
  * an existing Annotation Processor does what you want
    (E.g. Dagger, AutoService, etc.), or
  * you need to understand Java types to do generation.

### Custom Build Steps
* These use discrete build actions to generate source files.
  * Some generate `.java` directly, but most generate a zip file of sources
    (called a `.srcjar`) to simplify the number of inputs / outputs.
* Examples of existing templates:
  * `jinja_template`: Generates source files using [Jinja].
  * `java_cpp_template`: Generates source files using the C preprocessor.
  * `java_cpp_enum`: Generates `@IntDef`s based on enums within `.h` files.
  * `java_cpp_strings`: Generates String constants based on strings defined in
    `.cc` files.
* Custom build steps are preferred over Annotation Processors because they are
  generally easier to understand, and can run in parallel with other steps
  (rather than being tied to compiles).

[Jinja]: https://palletsprojects.com/p/jinja/

## Static Analysis & Code Checks

We use several tools for static analysis.

### [ErrorProne](https://errorprone.info/)
* Runs as part of normal compilation. Controlled by GN arg: `use_errorprone_java_compiler`.
* Most useful check:
  * Enforcement of `@GuardedBy` annotations.
* List of enabled / disabled checks exists [within javac.py](https://cs.chromium.org/chromium/src/build/android/gyp/javac.py?l=30)
  * Many checks are currently disabled because there is work involved in fixing
    violations they introduce. Please help!
* Custom checks for Chrome:
  * [//tools/android/errorprone_plugin/src/org/chromium/tools/errorprone/plugin/](/tools/android/errorprone_plugin/src/org/chromium/tools/errorprone/plugin/)
* Use ErrorProne checks when you need something more sophisticated than pattern
  matching.
* Checks run on the entire codebase, not only on changed lines.
* Does not run when `chromium_code = false` (e.g. for //third_party).

### [Android Lint](https://developer.android.com/studio/write/lint)
* Runs as part of normal compilation. Controlled by GN arg: `disable_android_lint` 
* Most useful check:
  * Enforcing `@TargetApi` annotations (ensure you don't call a function that
    does not exist on all versions of Android unless guarded by an version
    check).
* List of disabled checks:
  * [//build/android/lint/suppressions.xml](/build/android/lint/suppressions.xml)
* Custom lint checks [are possible](lint_plugins), but we don't have any.
* Checks run on the entire codebase, not only on changed lines.
* Does not run when `chromium_code = false` (e.g. for //third_party).

[lint_plugins]: http://tools.android.com/tips/lint-custom-rules

### [Bytecode Rewriter](/build/android/bytecode/)
* Runs as part of normal compilation.
* Performs a single check:
  * That target `deps` are not missing any entries.
  * In other words: Enforces that targets do not rely on indirect dependencies
    to populate their classpath.
* Checks run on the entire codebase, not only on changed lines.

### [PRESUBMIT.py](/PRESUBMIT.py):
* Checks for banned patterns via `_BANNED_JAVA_FUNCTIONS`.
  * (These should likely be moved to checkstyle).
* Checks for a random set of things in `_AndroidSpecificOnUploadChecks()`.
  * Including running Checkstyle.
  * (Some of these other checks should likely also be moved to checkstyle).
* Checks run only on changed lines.

### [Checkstyle](https://checkstyle.sourceforge.io/)
* Checks Java style rules that are not covered by clang-format.
  * E.g.: Unused imports and naming conventions.
* Allows custom checks to be added via XML. Here [is ours].
* Preferred over adding checks directly in PRESUBMIT.py because the tool
  understands `@SuppressWarnings` annotations.
* Checks run only on changed lines.

[is ours]:  /tools/android/checkstyle/chromium-style-5.0.xml

### [clang-format](https://clang.llvm.org/docs/ClangFormat.html)
* Formats `.java` files via `git cl format`.
* Can be toggle on/off with code comments.
  ```java
  // clang-format off
  ... non-formatted code here ...
  // clang-format on
  ```
* Does not work great for multiple annotations or on some lambda expressions,
  but is generally agreed it is better than not having it at all.