# Chromium Java Style Guide

_For other languages, please see the [Chromium style
guides](https://chromium.googlesource.com/chromium/src/+/main/styleguide/styleguide.md)._

Chromium follows the [Android Open Source style
guide](http://source.android.com/source/code-style.html) unless an exception
is listed below.

You can propose changes to this style guide by sending an email to
`java@chromium.org`. Ideally, the list will arrive at some consensus and you can
request review for a change to this file. If there's no consensus,
[`//styleguide/java/OWNERS`](https://chromium.googlesource.com/chromium/src/+/main/styleguide/java/OWNERS)
get to decide.

[TOC]

## Java 10 Language Features

### Type Deduction using `var`

A variable declaration can use the `var` keyword in place of the type (similar
to the `auto` keyword in C++). In line with the [guidance for
C++](https://google.github.io/styleguide/cppguide.html#Type_deduction), the
`var` keyword may be used when it aids readability and the type of the value is
already clear (ex. `var bundle = new Bundle()` is OK, but `var something =
returnValueIsNotObvious()` may be unclear to readers who are new to this part of
the code).

The `var` keyword may also be used in try-with-resources when the resource is
not directly accessed (or when it falls under the previous guidance), such as:

```java
try (var ignored = StrictModeContext.allowDiskWrites()) {
    // 'var' is permitted so long as the 'ignored' variable is not used directly
    // in the code.
}
```

## Java 8 Language Features

[D8] is used to rewrite some Java 7 & 8 language constructs in a way that is
compatible with Java 6 (and thus all Android versions). Use of [these features]
is encouraged.

[D8]: https://developer.android.com/studio/command-line/d8
[these features]: https://developer.android.com/studio/write/java8-support

## Java Library APIs

Android provides the ability to bundle copies of `java.` APIs alongside
application code, known as [Java Library Desugaring]. However, since this
bundling comes with a performance cost, Chrome does not use it. Treat `java.`
APIs the same as you would `android.` ones and guard them with
`Build.VERSION.SDK_INT` checks [when necessary]. The one exception is if the
method is [directly backported by D8] (these are okay to use, since they are
lightweight). Android Lint will fail if you try to use an API without a
corresponding `Build.VERSION.SDK_INT` guard or `@RequiresApi` annotation.

[Java Library Desugaring]: https://developer.android.com/studio/write/java8-support-table
[when necessary]: https://developer.android.com/reference/packages
[directly backported by D8]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/r8/backported_methods.txt

## Other Language Features & APIs

### Exceptions
We discourage overly broad catches via `Throwable`, `Exception`, or
`RuntimeException`, except when dealing with `RemoteException` or similar
system APIs.
 * There have been many cases of crashes caused by `IllegalStateException` /
   `IllegalArgumentException` / `SecurityException` being thrown where only
   `RemoteException` was being caught. In these cases, use
   `catch (RemoteException | RuntimeException e)`.
 * For all broad catch expressions, add a comment to explain why.

Avoid adding messages to exceptions that do not aid in debugging. For example:

```java
try {
  somethingThatThrowsIOException();
} catch (IOException e) {
  // Bad - message does not tell you more than the stack trace does:
  throw new RuntimeException("Failed to parse a file.", e);
  // Good - conveys that this block failed along with the "caused by" exception.
  throw new RuntimeException(e);
  // Good - adds useful information.
  throw new RuntimeException(String.format("Failed to parse %s", fileName), e);
}
```

### Logging

* Use `org.chromium.base.Log` instead of `android.util.Log`.
  * It provides `%s` support, and ensures log stripping works correctly.
* Minimize the use of `Log.w()` and `Log.e()`.
  * Debug and Info log levels are stripped by ProGuard in release builds, and
    so have no performance impact for shipping builds. However, Warning and
    Error log levels are not stripped.
* Function calls in log parameters are *not* stripped by ProGuard.

```java
Log.d(TAG, "There are %d cats", countCats());  // countCats() not stripped.
```

### Asserts

The build system:
 * strips asserts in release builds (via R8),
 * enables them in debug builds,
 * and enables them in report-only mode for Canary builds.

```java
// Code for assert expressions & messages is removed when asserts are disabled.
assert someCallWithoutSideEffects(param) : "Call failed with: " + param;
```

Use your judgement for when to use asserts vs exceptions. Generally speaking,
use asserts to check program invariants (e.g. parameter constraints) and
exceptions for unrecoverable error conditions (e.g. OS errors). You should tend
to use exceptions more in privacy / security-sensitive code.

Do not add checks when the code will crash anyways. E.g.:

```java
// Don't do this.
assert(foo != null);
foo.method(); // This will throw anyways.
```

For multi-statement asserts, use [`BuildConfig.ENABLE_ASSERTS`] to guard your
code (similar to `#if DCHECK_IS_ON()` in C++). E.g.:

```java
import org.chromium.build.BuildConfig;

...

if (BuildConfig.ENABLE_ASSERTS) {
  // Any code here will be stripped in release builds by R8.
  ...
}
```

[`BuildConfig.ENABLE_ASSERTS`]: https://source.chromium.org/search?q=symbol:BuildConfig%5C.ENABLE_ASSERTS

#### DCHECKS vs Java Asserts

`DCHECK` and `assert` are similar, but our guidance for them differs:
 * CHECKs are preferred in C++, whereas asserts are preferred in Java.

This is because as a memory-safe language, logic bugs in Java are much less
likely to be exploitable.

### Streams

Most uses of [Java 8 streams] are discouraged. If you can write your code as an
explicit loop, then do so. The primary reason for this guidance is because the
lambdas (and method references) needed for streams almost always result in
larger binary size ([example](https://chromium-review.googlesource.com/c/chromium/src/+/4329952).

The `parallel()` and `parallelStream()` APIs are simpler than their loop
equivalents, but are are currently banned due to a lack of a compelling use case
in Chrome. If you find one, please discuss on `java@chromium.org`.

[Java 8 streams]: https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html

### Finalizers

In line with [Google's Java style guide](https://google.github.io/styleguide/javaguide.html#s6.4-finalizers),
never override `Object.finalize()`.

Custom finalizers:
* are called on a background thread, and at an unpredicatble point in time,
* swallow all exceptions (asserts won't work),
* causes additional garbage collector jank.

Classes that need destructor logic should provide an explicit `destroy()`
method. Use [LifetimeAssert](https://chromium.googlesource.com/chromium/src/+/main/base/android/java/src/org/chromium/base/LifetimeAssert.java)
to ensure in debug builds and tests that `destroy()` is called.

### AndroidX Annotations

* Use them! They are [documented here](https://developer.android.com/studio/write/annotations).
  * They generally improve readability.
  * Some make lint more useful.
* `javax.annotation.Nullable` vs `androidx.annotation.Nullable`
  * Always prefer `androidx.annotation.Nullable`.
  * It uses `@Retention(SOURCE)` rather than `@Retention(RUNTIME)`.

### IntDef Instead of Enum

Java enums generate far more bytecode than integer constants. When integers are
sufficient, prefer using an [@IntDef annotation], which will have usage checked
by [Android lint].

Values can be declared outside or inside the `@interface`. We recommend the
latter, with constants nested within it as follows:

```java
@IntDef({ContactsPickerAction.CANCEL, ContactsPickerAction.CONTACTS_SELECTED,
        ContactsPickerAction.SELECT_ALL, ContactsPickerAction.UNDO_SELECT_ALL})
@Retention(RetentionPolicy.SOURCE)
public @interface ContactsPickerAction {
    int CANCEL = 0;
    int CONTACTS_SELECTED = 1;
    int SELECT_ALL = 2;
    int UNDO_SELECT_ALL = 3;
    int NUM_ENTRIES = 4;
}
// ...
void onContactsPickerUserAction(@ContactsPickerAction int action, ...);
```

Values of `Integer` type are also supported, which allows using a sentinel
`null` if needed.

[@IntDef annotation]: https://developer.android.com/studio/write/annotations#enum-annotations
[Android lint]: https://chromium.googlesource.com/chromium/src/+/HEAD/build/android/docs/lint.md

## Style / Formatting

### File Headers
* Use the same format as in the [C++ style guide](https://chromium.googlesource.com/chromium/src/+/main/styleguide/c++/c++.md#File-headers).

### TODOs

* TODO should follow chromium convention. Examples:
  * `TODO(username): Some sentence here.`
  * `TODO(crbug.com/123456): Even better to use a bug for context.`

### Code Formatting

* Fields should not be explicitly initialized to default values (see
  [here](https://groups.google.com/a/chromium.org/d/topic/chromium-dev/ylbLOvLs0bs/discussion)).

### Curly Braces

Conditional braces should be used, but are optional if the conditional and the
statement can be on a single line.

Do:

```java
if (someConditional) return false;
for (int i = 0; i < 10; ++i) callThing(i);
```

or

```java
if (someConditional) {
  return false;
}
```

Do NOT do:

```java
if (someConditional)
  return false;
```

### Import Order

* Static imports go before other imports.
* Each import group must be separated by an empty line.

This is the order of the import groups:

1. android
1. androidx
1. com (except com.google.android.apps.chrome)
1. dalvik
1. junit
1. org
1. com.google.android.apps.chrome
1. org.chromium
1. java
1. javax

## Testing

Googlers, see [go/clank-test-strategy](http://go/clank-test-strategy).

In summary:

* Use real dependencies when feasible and fast. Use Mockito’s `@Mock` most
  of the time, but write fakes for frequently used dependencies.

* Do not use Robolectric Shadows for Chromium code. Instead, use
  `setForTesting()` methods so that it is clear that test hooks exist.
  * When `setForTesting()` methods alter global state, use
    [`ResettersForTesting.register()`] to ensure that the state is reset
    between tests. Omit resetting them via `@After` methods.

* Use Robolectric when possible (when tests do not require native). Other
  times, use on-device tests with one of the following annotations:
  * [`@Batch(UNIT_TESTS)`] for unit tests
  * [`@Batch(PER_CLASS)`] for integration tests
  * [`@DoNotBatch`] for when each test method requires an app restart

[`ResettersForTesting.register()`]: https://source.chromium.org/search?q=symbol:ResettersForTesting.register
[`@Batch(UNIT_TESTS)`]: https://source.chromium.org/search?q=symbol:Batch.UNIT_TESTS
[`@Batch(PER_CLASS)`]: https://source.chromium.org/search?q=symbol:Batch.PER_CLASS
[`@DoNotBatch`]: https://source.chromium.org/search?q=symbol:DoNotBatch

### Test-only Code

Functions and fields used only for testing should have `ForTesting` as a
suffix so that:

1. The `android-binary-size` trybot can [ensure they are removed] in
   non-test optimized builds (by R8).
2. [`PRESUMBIT.py`] can ensure no calls are made to such methods outside of
   tests, and

`ForTesting` methods that are `@CalledByNative` should use
`@CalledByNativeForTesting` instead.

Symbols that are made public (or package-private) for the sake of tests
should be annotated with [`@VisibleForTesting`]. Android Lint will check
that calls from non-test code respect the "otherwise" visibility.

Symbols with a `ForTesting` suffix **should not** be annotated with
`@VisibleForTesting`. While `otherwise=VisibleForTesting.NONE` exists, it
is redundant given the "ForTesting" suffix and the associated lint check
is redundant given our trybot check. You should, however, use it for
test-only constructors.

[ensure they are removed]: /docs/speed/binary_size/android_binary_size_trybot.md#Added-Symbols-named-ForTest
[`PRESUMBIT.py`]: https://chromium.googlesource.com/chromium/src/+/main/PRESUBMIT.py
[`@VisibleForTesting`]: https://developer.android.com/reference/androidx/annotation/VisibleForTesting

## Location

"Top level directories" are defined as directories with a GN file, such as
[//base](https://chromium.googlesource.com/chromium/src/+/main/base/)
and
[//content](https://chromium.googlesource.com/chromium/src/+/main/content/),
Chromium Java should live in a directory named
`<top level directory>/android/java`, with a package name
`org.chromium.<top level directory>`.  Each top level directory's Java should
build into a distinct JAR that honors the abstraction specified in a native
[checkdeps](https://chromium.googlesource.com/chromium/buildtools/+/main/checkdeps/checkdeps.py)
(e.g. `org.chromium.base` does not import `org.chromium.content`).  The full
path of any java file should contain the complete package name.

For example, top level directory `//base` might contain a file named
`base/android/java/org/chromium/base/Class.java`. This would get compiled into a
`chromium_base.jar` (final JAR name TBD).

`org.chromium.chrome.browser.foo.Class` would live in
`chrome/android/java/org/chromium/chrome/browser/foo/Class.java`.

New `<top level directory>/android` directories should have an `OWNERS` file
much like
[//base/android/OWNERS](https://chromium.googlesource.com/chromium/src/+/main/base/android/OWNERS).

## Tools

### Automatically Formatting Edited Files

A checkout should give you clang-format to automatically format Java code.
It is suggested that Clang's formatting of code should be accepted in code
reviews.

You can run `git cl format` to apply the automatic formatting.

### IDE Setup

For automatically using the correct style, follow the guide to set up your
favorite IDE:

* [Android Studio](https://chromium.googlesource.com/chromium/src/+/main/docs/android_studio.md)
* [Eclipse](https://chromium.googlesource.com/chromium/src/+/main/docs/eclipse.md)

### Checkstyle

Checkstyle is automatically run by the build bots, and to ensure you do not have
any surprises, you can also set up checkstyle locally using [this
guide](https://sites.google.com/a/chromium.org/dev/developers/checkstyle).

### Lint

Lint is run as part of the build. For more information, see
[here](https://chromium.googlesource.com/chromium/src/+/main/build/android/docs/lint.md).

## Miscellany

* Use UTF-8 file encodings and LF line endings.