Add JPMS module test

Currently the build would not detected if `module-info.class` was missing,
or incorrectly configured.

.github/workflows/check-android-compatibility.yml

@@ -26,4 +26,5 @@ jobs:
       - name: Check Android compatibility
         run: |
           # Run 'test' phase because plugin normally expects to be executed after tests have been compiled
-          mvn --batch-mode --no-transfer-progress test animal-sniffer:check@check-android-compatibility -DskipTests
+          # Have to skip 'jpms-test' module because it requires that full Gson JAR has been built
+          mvn --batch-mode --no-transfer-progress test animal-sniffer:check@check-android-compatibility -DskipTests --projects '!jpms-test'

graal-native-image-test/pom.xml

@@ -23,6 +23,7 @@
     <version>2.11.1-SNAPSHOT</version>
   </parent>
   <artifactId>graal-native-image-test</artifactId>
+  <name>Test: GraalVM Native Image</name>
 
   <properties>
     <!-- Make the build reproducible, see root `pom.xml` -->

gson/src/main/java/module-info.java

@@ -25,7 +25,7 @@
   exports com.google.gson.reflect;
   exports com.google.gson.stream;
 
-  // Dependency on Error Prone Annotations
+  // Optional dependency on Error Prone Annotations
   requires static com.google.errorprone.annotations;
 
   // Optional dependency on java.sql

jpms-test/README.md

@@ -0,0 +1,5 @@
+# jpms-test
+
+This Maven module contains tests to verify that Gson's `module-info.class` which is used by the Java Platform Module System (JPMS) works properly and can be used by other projects. The module declaration file `src/test/java/module-info.java` uses Gson's module.
+
+This is a separate Maven module to test Gson's final JAR instead of just the compiled classes.

jpms-test/pom.xml

@@ -0,0 +1,110 @@
+<!--
+  Copyright 2024 Google Inc.
+
+  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.
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+
+  <parent>
+    <groupId>com.google.code.gson</groupId>
+    <artifactId>gson-parent</artifactId>
+    <version>2.11.1-SNAPSHOT</version>
+  </parent>
+  <artifactId>jpms-test</artifactId>
+  <name>Test: Java Platform Module System (JPMS)</name>
+
+  <properties>
+    <maven.compiler.release>11</maven.compiler.release>
+    <maven.compiler.testRelease>${maven.compiler.release}</maven.compiler.testRelease>
+
+    <!-- Make the build reproducible, see root `pom.xml` -->
+    <!-- This is duplicated here because that is recommended by `artifact:check-buildplan` -->
+    <project.build.outputTimestamp>2024-05-19T18:54:10Z</project.build.outputTimestamp>
+  </properties>
+
+  <dependencies>
+    <dependency>
+      <groupId>com.google.code.gson</groupId>
+      <artifactId>gson</artifactId>
+      <version>${project.parent.version}</version>
+    </dependency>
+
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
+      <groupId>com.google.truth</groupId>
+      <artifactId>truth</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+
+  <build>
+    <pluginManagement>
+      <plugins>
+        <plugin>
+          <groupId>com.github.siom79.japicmp</groupId>
+          <artifactId>japicmp-maven-plugin</artifactId>
+          <configuration>
+            <!-- This module is not supposed to be consumed as library, so no need to check API -->
+            <skip>true</skip>
+          </configuration>
+        </plugin>
+        <plugin>
+          <groupId>org.codehaus.mojo</groupId>
+          <artifactId>animal-sniffer-maven-plugin</artifactId>
+          <configuration>
+            <!-- This module is not supposed to be consumed as library, so no need to check used classes -->
+            <skip>true</skip>
+          </configuration>
+        </plugin>
+        <plugin>
+          <groupId>org.apache.maven.plugins</groupId>
+          <artifactId>maven-jar-plugin</artifactId>
+          <configuration>
+            <!-- This module has no 'main' source code; skip creating JAR and avoid warning on console -->
+            <skipIfEmpty>true</skipIfEmpty>
+          </configuration>
+        </plugin>
+        <plugin>
+          <groupId>org.apache.maven.plugins</groupId>
+          <artifactId>maven-install-plugin</artifactId>
+          <configuration>
+            <!-- This module has no 'main' source code, so no JAR is created which could be installed;
+              see maven-jar-plugin configuration above -->
+            <skip>true</skip>
+          </configuration>
+        </plugin>
+        <plugin>
+          <groupId>org.apache.maven.plugins</groupId>
+          <artifactId>maven-deploy-plugin</artifactId>
+          <configuration>
+            <!-- Not deployed -->
+            <skip>true</skip>
+          </configuration>
+        </plugin>
+        <plugin>
+          <groupId>org.apache.maven.plugins</groupId>
+          <artifactId>maven-gpg-plugin</artifactId>
+          <configuration>
+            <!-- Not deployed -->
+            <skip>true</skip>
+          </configuration>
+        </plugin>
+      </plugins>
+    </pluginManagement>
+  </build>
+</project>

jpms-test/src/main/java/module-info.java

@@ -0,0 +1,27 @@
+/*
+ * Copyright (C) 2024 Google Inc.
+ *
+ * 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.
+ */
+
+/**
+ * Dummy module to prevent Maven Compiler Plugin from failing if {@code module-info.java} exists
+ * only in test sources:
+ *
+ * <blockquote>
+ *
+ * Can't compile test sources when main sources are missing a module descriptor
+ *
+ * </blockquote>
+ */
+module com.google.gson.dummy {}

jpms-test/src/test/java/com/google/gson/jpms_test/ExportedPackagesTest.java

@@ -0,0 +1,100 @@
+/*
+ * Copyright (C) 2024 Google Inc.
+ *
+ * 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.gson.jpms_test;
+
+import static com.google.common.truth.Truth.assertThat;
+import static org.junit.Assert.assertThrows;
+
+import com.google.gson.Gson;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.reflect.TypeToken;
+import com.google.gson.stream.JsonReader;
+import java.io.IOException;
+import java.io.StringReader;
+import java.lang.reflect.Constructor;
+import java.lang.reflect.Field;
+import java.lang.reflect.InaccessibleObjectException;
+import java.lang.reflect.Modifier;
+import java.util.Arrays;
+import org.junit.Test;
+
+/**
+ * Verifies that Gson's {@code module-info.class} properly 'exports' all its packages containing
+ * public API.
+ */
+public class ExportedPackagesTest {
+  /** Tests package {@code com.google.gson} */
+  @Test
+  public void testMainPackage() {
+    Gson gson = new Gson();
+    assertThat(gson.toJson(1)).isEqualTo("1");
+  }
+
+  /** Tests package {@code com.google.gson.annotations} */
+  @Test
+  public void testAnnotationsPackage() throws Exception {
+    class Annotated {
+      @SerializedName("custom-name")
+      int i;
+    }
+
+    Field field = Annotated.class.getDeclaredField("i");
+    SerializedName annotation = field.getAnnotation(SerializedName.class);
+    assertThat(annotation.value()).isEqualTo("custom-name");
+  }
+
+  /** Tests package {@code com.google.gson.reflect} */
+  @Test
+  public void testReflectPackage() {
+    var typeToken = TypeToken.get(String.class);
+    assertThat(typeToken.getRawType()).isEqualTo(String.class);
+  }
+
+  /** Tests package {@code com.google.gson.stream} */
+  @Test
+  public void testStreamPackage() throws IOException {
+    JsonReader jsonReader = new JsonReader(new StringReader("2"));
+    assertThat(jsonReader.nextInt()).isEqualTo(2);
+  }
+
+  /** Verifies that Gson packages are only 'exported' but not 'opened' for reflection. */
+  @Test
+  public void testReflectionInternalField() throws Exception {
+    Gson gson = new Gson();
+
+    // Get an arbitrary non-public instance field
+    Field field =
+        Arrays.stream(Gson.class.getDeclaredFields())
+            .filter(
+                f -> !Modifier.isStatic(f.getModifiers()) && !Modifier.isPublic(f.getModifiers()))
+            .findFirst()
+            .get();
+    assertThrows(InaccessibleObjectException.class, () -> field.setAccessible(true));
+    assertThrows(IllegalAccessException.class, () -> field.get(gson));
+  }
+
+  @Test
+  public void testInaccessiblePackage() throws Exception {
+    // Note: In case this class is renamed / removed, can change this to any other internal class
+    Class<?> internalClass = Class.forName("com.google.gson.internal.LinkedTreeMap");
+    assertThat(Modifier.isPublic(internalClass.getModifiers())).isTrue();
+    // Get the public constructor
+    Constructor<?> constructor = internalClass.getConstructor();
+    assertThrows(InaccessibleObjectException.class, () -> constructor.setAccessible(true));
+    assertThrows(IllegalAccessException.class, () -> constructor.newInstance());
+  }
+}

jpms-test/src/test/java/com/google/gson/jpms_test/ModuleTest.java

@@ -0,0 +1,114 @@
+/*
+ * Copyright (C) 2024 Google Inc.
+ *
+ * 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.gson.jpms_test;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.gson.Gson;
+import java.lang.module.ModuleDescriptor;
+import java.lang.module.ModuleDescriptor.Exports;
+import java.lang.module.ModuleDescriptor.Requires;
+import java.net.URL;
+import java.util.Set;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+import org.junit.Test;
+
+/**
+ * Verifies that this test project is properly set up and has a module descriptor, and checks Gson's
+ * module descriptor.
+ */
+public class ModuleTest {
+  @Test
+  public void testOwnModule() {
+    Module module = getClass().getModule();
+    assertThat(module.getName()).isEqualTo("com.google.gson.jpms_test");
+  }
+
+  @Test
+  public void testGsonModule() {
+    // Verify that this test actually loads the final Gson JAR, and not only compiled classes
+    // Note: This might fail when run from the IDE, but should succeed when run with Maven from
+    // command line
+    URL gsonLocation = Gson.class.getProtectionDomain().getCodeSource().getLocation();
+    assertThat(gsonLocation.getPath()).containsMatch("gson/target/gson-[^/]+\\.jar");
+
+    Module module = Gson.class.getModule();
+    ModuleDescriptor moduleDescriptor = module.getDescriptor();
+
+    assertThat(moduleDescriptor.name()).isEqualTo("com.google.gson");
+    // Permit `Modifier.SYNTHETIC`; current versions of Moditect seem to set that
+    assertThat(moduleDescriptor.modifiers())
+        .containsNoneOf(
+            ModuleDescriptor.Modifier.AUTOMATIC,
+            ModuleDescriptor.Modifier.MANDATED,
+            ModuleDescriptor.Modifier.OPEN);
+    // Should have implicitly included the Maven project version
+    assertThat(moduleDescriptor.rawVersion()).isPresent();
+
+    Set<Requires> moduleRequires = moduleDescriptor.requires();
+    assertThat(getModuleDependencies(moduleRequires))
+        .containsExactly("com.google.errorprone.annotations", "java.sql", "jdk.unsupported");
+    assertThat(getTransitiveModuleDependencies(moduleRequires)).isEmpty();
+    assertThat(getOptionalModuleDependencies(moduleRequires))
+        .containsExactly("com.google.errorprone.annotations", "java.sql", "jdk.unsupported");
+
+    Set<Exports> packageExports = moduleDescriptor.exports();
+    assertThat(packageExports.stream().map(Exports::source))
+        .containsExactly(
+            "com.google.gson",
+            "com.google.gson.annotations",
+            "com.google.gson.reflect",
+            "com.google.gson.stream");
+    // Gson currently does not export packages to specific modules only
+    assertThat(packageExports.stream().filter(Exports::isQualified)).isEmpty();
+
+    // Gson does not allow access to its implementation details using reflection
+    assertThat(moduleDescriptor.opens()).isEmpty();
+    // Gson currently does not use or provide any services
+    assertThat(moduleDescriptor.uses()).isEmpty();
+    assertThat(moduleDescriptor.provides()).isEmpty();
+    // Gson has no main class
+    assertThat(moduleDescriptor.mainClass()).isEmpty();
+  }
+
+  private static Stream<Requires> filterImplicitRequires(Set<Requires> requires) {
+    return requires.stream()
+        .filter(
+            r ->
+                !r.modifiers().contains(Requires.Modifier.MANDATED)
+                    && !r.modifiers().contains(Requires.Modifier.SYNTHETIC));
+  }
+
+  private static Set<String> getModuleDependencies(Set<Requires> requires) {
+    return filterImplicitRequires(requires).map(Requires::name).collect(Collectors.toSet());
+  }
+
+  private static Set<String> getTransitiveModuleDependencies(Set<Requires> requires) {
+    return filterImplicitRequires(requires)
+        .filter(r -> r.modifiers().contains(Requires.Modifier.TRANSITIVE))
+        .map(Requires::name)
+        .collect(Collectors.toSet());
+  }
+
+  private static Set<String> getOptionalModuleDependencies(Set<Requires> requires) {
+    return filterImplicitRequires(requires)
+        .filter(r -> r.modifiers().contains(Requires.Modifier.STATIC))
+        .map(Requires::name)
+        .collect(Collectors.toSet());
+  }
+}