Add @NonNullApi and annotate two packages with it.

This new annotation declares everything as non-null by default and can be
used as a package annotation in package-info.java.

In this change the core lib offline package and the mediasession extension is
annotated that way as initial example usage.

PiperOrigin-RevId: 260894548

constants.gradle

@@ -24,6 +24,7 @@ project.ext {
     autoValueVersion = '1.6'
     autoServiceVersion = '1.0-rc4'
     checkerframeworkVersion = '2.5.0'
+    jsr305Version = '3.0.2'
     androidXTestVersion = '1.1.0'
     truthVersion = '0.44'
     modulePrefix = ':'

extensions/mediasession/src/main/java/com/google/android/exoplayer2/ext/mediasession/package-info.java

@@ -0,0 +1,19 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+@NonNullApi
+package com.google.android.exoplayer2.ext.mediasession;
+
+import com.google.android.exoplayer2.util.NonNullApi;

library/core/build.gradle

@@ -59,8 +59,11 @@ android {
 
 dependencies {
     implementation 'androidx.annotation:annotation:1.0.2'
+    compileOnly 'com.google.code.findbugs:jsr305:' + jsr305Version
     compileOnly 'org.checkerframework:checker-qual:' + checkerframeworkVersion
     compileOnly 'org.checkerframework:checker-compat-qual:' + checkerframeworkVersion
+    // Uncomment to enable Kotlin non-null strict mode. See [internal: b/138703808].
+    // compileOnly "org.jetbrains.kotlin:kotlin-annotations-jvm:1.1.60"
     androidTestImplementation 'androidx.test:runner:' + androidXTestVersion
     androidTestImplementation 'androidx.test.ext:junit:' + androidXTestVersion
     androidTestImplementation 'com.google.truth:truth:' + truthVersion

library/core/src/main/java/com/google/android/exoplayer2/offline/package-info.java

@@ -0,0 +1,19 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+@NonNullApi
+package com.google.android.exoplayer2.offline;
+
+import com.google.android.exoplayer2.util.NonNullApi;

library/core/src/main/java/com/google/android/exoplayer2/util/NonNullApi.java

@@ -0,0 +1,36 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.android.exoplayer2.util;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import javax.annotation.Nonnull;
+import javax.annotation.meta.TypeQualifierDefault;
+// import kotlin.annotations.jvm.MigrationStatus;
+// import kotlin.annotations.jvm.UnderMigration;
+
+/**
+ * Annotation to declare all type usages in the annotated instance as {@link Nonnull}, unless
+ * explicitly marked with a nullable annotation.
+ */
+@Nonnull
+@TypeQualifierDefault(ElementType.TYPE_USE)
+// TODO(internal: b/138703808): Uncomment to ensure Kotlin issues compiler errors when non-null
+// types are used incorrectly.
+// @UnderMigration(status = MigrationStatus.STRICT)
+@Retention(RetentionPolicy.CLASS)
+public @interface NonNullApi {}