Chromium Java Style Guide

For other languages, please see the Chromium style guides.

Chromium follows the Android Open Source style guide 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 get to decide.

Java 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++, 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:

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

Exceptions

A quick primer:

• Throwable: Base class for all exceptions
  • Error: Base class for exceptions which are meant to crash the app.
  • Exception: Base class for exceptions that make sense the catch.
    • RuntimeException: Base class for exceptions that do not need to be declared as throws (“unchecked exceptions”).

Broad Catch Handlers

Use catch statements that do not catch exceptions they are not meant to.

• There is rarely a valid reason to catch (Throwable t), since that includes the (generally unrecoverable) Error types.

Use catch (Exception e) when working with OS APIs that might throw (assuming the program can recover from them).

• There have been many cases of crashes caused by IllegalStateException / IllegalArgumentException / SecurityException being thrown where only RemoteException was being caught. Unless catch handlers will differ based on exception type, just catch Exception.

Do not use catch (RuntimeException e).

• It is useful to extend RuntimeException to make unchecked exception types, but the type does not make much sense in catch clauses, as there are not times when you'd want to catch all unchecked exceptions, but not also want to catch all checked exceptions.

Exception Messages

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

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);
}

Wrapping with RuntimeException

It is common to wrap a checked exception with a RuntimeException for cases where a checked exception is not recoverable, or not possible. In order to reduce the number of stack trace “caused by” clauses, and to save on binary size, use JavaUtils.throwUnchecked() instead.

try {
    somethingThatThrowsIOException();
} catch (IOException e) {
    // Bad - RuntimeException adds no context and creates longer stack traces.
    throw new RuntimeException(e);
    // Good - Original exception is preserved.
    throw JavaUtils.throwUnchecked(e);
}

Do not use throwUnchecked() when the exception may want to be caught.

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.

// 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.:

// 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.:

import org.chromium.build.BuildConfig;

...

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

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.

toString()

Use explicit serialization methods (e.g. toDebugString() or getDescription()) instead of toString() when dynamic dispatch is not required.

1. R8 cannot detect when toString() is unused, so overrides will not be stripped when unused.
2. R8 cannot optimize / inline these calls as well as non-overriding methods.

Records & AutoValue

// Banned.
record Rectangle(float length, float width) {}

Rationale:

• To avoid dead code:
  • Records and @AutoValue generate equals(), hashCode(), and toString(), which R8 is unable to remove when unused.
  • When these methods are required, implement them explicitly so that the intention is clear.
• Also - supporting record requires build system work (crbug/1493366).

Example with equals() and hashCode():

public class ValueClass {
    private final SomeClass mObjMember;
    private final int mIntMember;

    @Override
    public boolean equals(Object o) {
        return o instanceof ValueClass vc
                && Objects.equals(mObjMember, vc.mObjMember)
                && mIntMember == vc.mIntMember;
    }

    @Override
    public int hashCode() {
        return Objects.hash(mObjMember, mIntMember);
    }
}

Enums

Banned. Use @IntDef instead.

Rationale:

Java enums generate a lot of bytecode. Use constants where possible. When a custom type hierarchy is required, use explicit classes with inheritance.

Finalizers

In line with Google's Java style guide and Android's Java style guide, 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 to ensure in debug builds and tests that destroy() is called.

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.

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.

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

Streams

Most uses of Java 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.

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.

AndroidX Annotations

• Use them liberally. They are documented here.
  • They generally improve readability.
  • Many make lint more useful.
• javax.annotation.Nullable vs androidx.annotation.Nullable
  • Always prefer androidx.annotation.Nullable.
  • It uses @Retention(SOURCE) rather than @Retention(RUNTIME).

IntDefs

Values can be declared outside or inside the @interface. Chromium style is to declare inside.

@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.

Style / Formatting

File Headers

• Use the same format as in the C++ style guide.

TODOs

• TODO should follow chromium convention. Examples:
  • TODO(username): Some sentence here.
  • TODO(crbug.com/40192027): Even better to use a bug for context.

Parameter Comments

Use parameter comments when they aid in the readability of a function call.

E.g.:

someMethod(/* enabled= */ true, /* target= */ null, defaultValue);

Default Field Initializers

• Fields should not be explicitly initialized to default values (see here).

Curly Braces

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

Do:

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

or

if (someConditional) {
    return false;
}

Do NOT do:

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
2. androidx
3. com (except com.google.android.apps.chrome)
4. dalvik
5. junit
6. org
7. com.google.android.apps.chrome
8. org.chromium
9. java
10. javax

Testing

Googlers, see 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.

  • Shadows make code harder to refactor.
  • Prefer to refactor code to make it more testable.
  • When you really need to use a test double for a static method, add a setFooForTesting() [...] method to make the test contract explicit.
    • Use ResettersForTesting.register() from within ForTesting() methods to ensure that state is reset between tests.

• 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

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.

Location

“Top level directories” are defined as directories with a GN file, such as //base and //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 (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.

Tools

google-java-format is used to auto-format Java files. Formatting of its code should be accepted in code reviews.

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

Chromium also makes use of several static analysis tools.

Miscellany

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