fix: disallow non-static inner classes

fixes: #2746

examples/java/echo/ftl.toml

@@ -0,0 +1,2 @@
+module = "echo"
+language = "kotlin"

examples/java/echo/pom.xml

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<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>
+    <groupId>xyz.block.ftl.examples</groupId>
+    <artifactId>echo</artifactId>
+    <version>1.0-SNAPSHOT</version>
+
+    <parent>
+        <groupId>xyz.block.ftl</groupId>
+        <artifactId>ftl-build-parent-java</artifactId>
+        <version>1.0-SNAPSHOT</version>
+    </parent>
+
+</project>

examples/java/echo/src/main/java/ftl/echo/Echo.java

@@ -0,0 +1,15 @@
+package ftl.echo;
+
+import ftl.time.TimeClient;
+import xyz.block.ftl.Export;
+import xyz.block.ftl.Verb;
+
+public class Echo {
+
+    @Export
+    @Verb
+    public EchoResponse echo(EchoRequest req, TimeClient time) {
+        var response = time.call();
+        return new EchoResponse("Hello, " + req.name().orElse("anonymous") + "! The time is " + response.toString() + ".");
+    }
+}

examples/java/echo/src/main/java/ftl/echo/EchoRequest.java

@@ -0,0 +1,6 @@
+package ftl.echo;
+
+import java.util.Optional;
+
+public record EchoRequest(Optional<String> name) {
+}

examples/java/echo/src/main/java/ftl/echo/EchoResponse.java

@@ -0,0 +1,4 @@
+package ftl.echo;
+
+public record EchoResponse(String message) {
+}

examples/java/time/ftl.toml

@@ -0,0 +1,2 @@
+module = "time"
+language = "kotlin"

examples/java/time/pom.xml

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<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>
+    <groupId>xyz.block.ftl.examples</groupId>
+    <artifactId>time</artifactId>
+    <version>1.0-SNAPSHOT</version>
+
+    <parent>
+        <groupId>xyz.block.ftl</groupId>
+        <artifactId>ftl-build-parent-java</artifactId>
+        <version>1.0-SNAPSHOT</version>
+    </parent>
+
+</project>

examples/java/time/src/main/java/ftl/time/Time.java

@@ -0,0 +1,15 @@
+package ftl.time;
+
+import java.time.OffsetDateTime;
+
+import xyz.block.ftl.Export;
+import xyz.block.ftl.Verb;
+
+public class Time {
+
+    @Verb
+    @Export
+    public static TimeResponse time() {
+        return new TimeResponse(OffsetDateTime.now());
+    }
+}

examples/java/time/src/main/java/ftl/time/TimeResponse.java

@@ -0,0 +1,6 @@
+package ftl.time;
+
+import java.time.OffsetDateTime;
+
+public record TimeResponse(OffsetDateTime time) {
+}

jvm-runtime/ftl-runtime/common/deployment/src/main/java/xyz/block/ftl/deployment/ModuleBuilder.java

@@ -85,6 +85,7 @@ public class ModuleBuilder {
     private final Map<DotName, VerbClientBuildItem.DiscoveredClients> verbClients;
     private final FTLRecorder recorder;
     private final Map<String, String> verbDocs;
+    private final List<ValidationFailure> validationFailures = new ArrayList<>();
 
     public ModuleBuilder(IndexView index, String moduleName, Map<DotName, TopicsBuildItem.DiscoveredTopic> knownTopics,
             Map<DotName, VerbClientBuildItem.DiscoveredClients> verbClients, FTLRecorder recorder,
@@ -294,6 +295,11 @@ public Type buildType(org.jboss.jandex.Type type, boolean export) {
             case CLASS -> {
                 var clazz = type.asClassType();
                 var info = index.getClassByName(clazz.name());
+                if (info.enclosingClass() != null && !Modifier.isStatic(info.flags())) {
+                    // proceed as normal, we fail at the end
+                    validationFailures.add(new ValidationFailure(clazz.name().toString(),
+                            "Inner classes must be static"));
+                }
 
                 PrimitiveType unboxed = PrimitiveType.unbox(clazz);
                 if (unboxed != null) {
@@ -435,6 +441,14 @@ public ModuleBuilder addDecls(Decl decl) {
     }
 
     public void writeTo(OutputStream out) throws IOException {
+        if (!validationFailures.isEmpty()) {
+            StringBuilder sb = new StringBuilder();
+            for (var failure : validationFailures) {
+                sb.append("Validation failure: ").append(failure.className).append(": ").append(failure.message)
+                        .append("\n");
+            }
+            throw new RuntimeException(sb.toString());
+        }
         moduleBuilder.build().writeTo(out);
     }
 
@@ -456,4 +470,7 @@ public enum BodyType {
         ALLOWED,
         REQUIRED
     }
+
+    record ValidationFailure(String className, String message) {
+    }
 }