Support Java codegen for the OpenAPI backend (pantsbuild#16862)

Initial implementation for a codegen feature in the OpenAPI backend

src/python/pants/backend/experimental/openapi/codegen/java/BUILD

@@ -0,0 +1,4 @@
+# Copyright 2022 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+python_sources()

src/python/pants/backend/experimental/openapi/codegen/java/register.py

@@ -0,0 +1,15 @@
+# Copyright 2022 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import annotations
+
+from pants.backend.experimental.java.register import rules as java_rules
+from pants.backend.openapi.codegen.java.rules import rules as java_codegen_rules
+
+
+def target_types():
+    return []
+
+
+def rules():
+    return [*java_rules(), *java_codegen_rules()]

src/python/pants/backend/openapi/codegen/java/BUILD

@@ -0,0 +1,8 @@
+# Copyright 2022 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+python_sources()
+
+python_tests(name="tests", dependencies=[":lockfiles"])
+
+resources(name="lockfiles", sources=["*.test.lock"])

src/python/pants/backend/openapi/codegen/java/extra_fields.py

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+# Copyright 2022 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from pants.backend.openapi.target_types import OpenApiDocumentGeneratorTarget, OpenApiDocumentTarget
+from pants.engine.target import BoolField, StringField
+from pants.jvm.target_types import PrefixedJvmJdkField, PrefixedJvmResolveField
+
+
+class OpenApiJavaModelPackageField(StringField):
+    alias = "java_model_package"
+    help = "Root package for generated model code"
+
+
+class OpenApiJavaApiPackageField(StringField):
+    alias = "java_api_package"
+    help = "Root package for generated API code"
+
+
+class OpenApiJavaSkipField(BoolField):
+    alias = "skip_java"
+    default = False
+    help = "If true, skips generation of Java sources from this target"
+
+
+def rules():
+    return [
+        OpenApiDocumentTarget.register_plugin_field(OpenApiJavaSkipField),
+        OpenApiDocumentTarget.register_plugin_field(OpenApiJavaModelPackageField),
+        OpenApiDocumentTarget.register_plugin_field(OpenApiJavaApiPackageField),
+        OpenApiDocumentGeneratorTarget.register_plugin_field(OpenApiJavaSkipField),
+        OpenApiDocumentGeneratorTarget.register_plugin_field(OpenApiJavaModelPackageField),
+        OpenApiDocumentGeneratorTarget.register_plugin_field(OpenApiJavaApiPackageField),
+        # Default Pants JVM fields
+        OpenApiDocumentTarget.register_plugin_field(PrefixedJvmJdkField),
+        OpenApiDocumentTarget.register_plugin_field(PrefixedJvmResolveField),
+        OpenApiDocumentGeneratorTarget.register_plugin_field(PrefixedJvmJdkField),
+        OpenApiDocumentGeneratorTarget.register_plugin_field(PrefixedJvmResolveField),
+    ]