feat(gapic-generator): add JSpecify Null annotations to the generator classes (#13769)

This PR continues the onboarding of the gapic-generator-java project to
JSpecify null-safety. Specifically, it applies the @NullMarked
annotation to all modified source and test classes in this branch to
declare them non-null by default.

**Annotations Applied**
- @NullMarked Class Declarations: Applied at the class level to all
modified source and test classes to establish a non-null by default
context.
- @nullable Type Annotations: Applied to specific methods, fields, and
parameters to explicitly denote where null values are accepted/returned.

**Automation Method**
To apply the class-level annotations efficiently, we leveraged Chris's
Error Prone automation tool (custom BugChecker plugin under
com.google.errorprone.bugpatterns.nullness) to run AST-based refactoring
and automatically insert the JSpecify @NullMarked annotations across the
codebase.

Verification/Testing
- Please refer to this doc:
https://docs.google.com/document/d/18c7NzbTpREqqb_qvLUZ1bCaMYGQQnczyo6vrVPmx60Q/edit?resourcekey=0-EOZQxK66NvVhJPF1jg-LQA&tab=t.0

Author: nnicolee

sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/Main.java

@@ -21,13 +21,16 @@
 import com.google.protobuf.compiler.PluginProtos.CodeGeneratorRequest;
 import com.google.protobuf.compiler.PluginProtos.CodeGeneratorResponse;
 import java.io.IOException;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 public class Main {
   public static void main(String[] args) throws IOException {
     ExtensionRegistry registry = ExtensionRegistry.newInstance();
     ProtoRegistry.registerAllExtensions(registry);
     CodeGeneratorRequest request = CodeGeneratorRequest.parseFrom(System.in, registry);
-    CodeGeneratorResponse response = Generator.generateGapic(request);
+    @Nullable CodeGeneratorResponse response = Generator.generateGapic(request);
     if (response != EMPTY_RESPONSE) {
       response.writeTo(System.out);
     }

sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/ProtoRegistry.java

@@ -23,7 +23,9 @@
 import com.google.cloud.ExtendedOperationsProto;
 import com.google.longrunning.OperationsProto;
 import com.google.protobuf.ExtensionRegistry;
+import org.jspecify.annotations.NullMarked;
 
+@NullMarked
 public class ProtoRegistry {
   /** Register all extensions needed to process API protofiles. */
   public static void registerAllExtensions(ExtensionRegistry extensionRegistry) {

sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/debug/CodeGeneratorRequestDumper.java

@@ -19,12 +19,14 @@
 import com.google.protobuf.compiler.PluginProtos.CodeGeneratorRequest;
 import com.google.protobuf.compiler.PluginProtos.CodeGeneratorResponse;
 import java.io.IOException;
+import org.jspecify.annotations.NullMarked;
 
 // Request dumper class, which dumps the CodeGeneratorRequest to a file on disk which will be
 // identical to the one passed to the Main class during normal execution. The dumped file then can
 // be used to run this gapic-generator directly (instead of relying on protoc to start the process),
 // which would give much greater flexibility in terms of debugging features, like attaching a
 // debugger, easier work with stdout and stderr etc.
+@NullMarked
 public class CodeGeneratorRequestDumper {
   public static void main(String[] args) throws IOException {
     ExtensionRegistry registry = ExtensionRegistry.newInstance();

sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/debug/CodeGeneratorRequestFileToGapicMain.java

@@ -24,9 +24,12 @@
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
 // A generator entry point class, similar to Main but reads the CodeGeneratorRequest directly from a
 // file instead of relying on protoc to pipe it in.
+@NullMarked
 public class CodeGeneratorRequestFileToGapicMain {
   public static void main(String[] args) throws IOException {
     ExtensionRegistry registry = ExtensionRegistry.newInstance();
@@ -38,7 +41,7 @@ public static void main(String[] args) throws IOException {
     try (InputStream inputStream = new FileInputStream(inputFile);
         OutputStream outputStream = new FileOutputStream(outputFile)) {
       CodeGeneratorRequest request = CodeGeneratorRequest.parseFrom(inputStream, registry);
-      CodeGeneratorResponse response = Generator.generateGapic(request);
+      @Nullable CodeGeneratorResponse response = Generator.generateGapic(request);
       response.writeTo(outputStream);
     }
   }

sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/AnnotationNode.java

@@ -19,8 +19,10 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class AnnotationNode implements AstNode {
   public static AnnotationNode OVERRIDE =
@@ -34,8 +36,7 @@ private static TypeNode annotationType(Class<?> clazz) {
 
   public abstract TypeNode type();
 
-  @Nullable
-  public abstract List<Expr> descriptionExprs();
+  public abstract @Nullable List<Expr> descriptionExprs();
 
   @Override
   public void accept(AstNodeVisitor visitor) {

sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/AnonymousClassExpr.java

@@ -20,7 +20,9 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
+import org.jspecify.annotations.NullMarked;
 
+@NullMarked
 @AutoValue
 public abstract class AnonymousClassExpr implements Expr {
   @Override

sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ArithmeticOperationExpr.java

@@ -16,7 +16,9 @@
 
 import com.google.auto.value.AutoValue;
 import com.google.common.base.Preconditions;
+import org.jspecify.annotations.NullMarked;
 
+@NullMarked
 @AutoValue
 public abstract class ArithmeticOperationExpr implements OperationExpr {
 

sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ArrayExpr.java

@@ -19,7 +19,9 @@
 import com.google.common.collect.ImmutableList;
 import java.util.Arrays;
 import java.util.List;
+import org.jspecify.annotations.NullMarked;
 
+@NullMarked
 @AutoValue
 public abstract class ArrayExpr implements Expr {
 

sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/AssignmentExpr.java

@@ -16,7 +16,9 @@
 
 import com.google.auto.value.AutoValue;
 import com.google.common.base.Preconditions;
+import org.jspecify.annotations.NullMarked;
 
+@NullMarked
 @AutoValue
 public abstract class AssignmentExpr implements Expr {
   public abstract VariableExpr variableExpr();

sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/AssignmentOperationExpr.java

@@ -16,7 +16,9 @@
 
 import com.google.auto.value.AutoValue;
 import com.google.common.base.Preconditions;
+import org.jspecify.annotations.NullMarked;
 
+@NullMarked
 @AutoValue
 public abstract class AssignmentOperationExpr implements OperationExpr {
   public abstract VariableExpr variableExpr();