Support Javalin async request handling [Context#future(...)]

http-generator-core/src/main/java/io/avaje/http/generator/core/JsonBUtil.java

@@ -50,6 +50,12 @@ private static void addJsonBodyType(MethodReader methodReader, Consumer<UType> a
   }
 
   public static void writeJsonbType(UType type, Append writer) {
+    // Support for CompletableFuture's.
+    if (type.mainType().equals("java.util.concurrent.CompletableFuture")) {
+      writeJsonbType(type.paramRaw(), writer);
+      return;
+    }
+
     writer.append("    this.%sJsonType = jsonB.type(", type.shortName());
     if (!type.isGeneric()) {
       writer.append("%s.class)", Util.shortName(PrimitiveUtil.wrap(type.full())));

http-generator-javalin/src/main/java/io/avaje/http/generator/javalin/ControllerMethodWriter.java

@@ -92,20 +92,47 @@ void write(boolean requestScoped) {
   }
 
   private void writeContextReturn() {
+    // Support for CompletableFuture's.
+    final UType type = UType.parse(method.returnType());
+    if (type.mainType().equals("java.util.concurrent.CompletableFuture")) {
+      if (!type.isGeneric()) {
+        throw new IllegalStateException("CompletableFuture must be generic type (e.g. CompletableFuture<String>, CompletableFuture<Void>).");
+      }
+
+      final String futureResultVariableName = "futureResult";
+
+      writer.append("      ctx.future(() -> {").eol();
+      writer.append("        return result.thenAccept(%s -> {", futureResultVariableName).eol();
+      writer.append("    ");
+      this.writeContextReturn(futureResultVariableName);
+      writer.eol().append("        });").eol();
+      writer.append("      });").eol();
+      return;
+    }
+
+    // Everything else
+    this.writeContextReturn("result");
+  }
+
+  private void writeContextReturn(final String resultVariableName) {
     final var produces = method.produces();
     if (produces == null || MediaType.APPLICATION_JSON.getValue().equalsIgnoreCase(produces)) {
       if (useJsonB) {
-        final var uType = UType.parse(method.returnType());
-        writer.append("      %sJsonType.toJson(result, ctx.contentType(\"application/json\").outputStream());", uType.shortName());
+        var uType = UType.parse(method.returnType());
+        if (uType.mainType().equals("java.util.concurrent.CompletableFuture")) {
+          uType = uType.paramRaw();
+        }
+
+        writer.append("      %sJsonType.toJson(%s, ctx.contentType(\"application/json\").outputStream());", uType.shortName(), resultVariableName);
       } else {
-        writer.append("      ctx.json(result);");
+        writer.append("      ctx.json(%s);", resultVariableName);
       }
     } else if (MediaType.TEXT_HTML.getValue().equalsIgnoreCase(produces)) {
-      writer.append("      ctx.html(result);");
+      writer.append("      ctx.html(%s);", resultVariableName);
     } else if (MediaType.TEXT_PLAIN.getValue().equalsIgnoreCase(produces)) {
-      writer.append("      ctx.contentType(\"text/plain\").result(result);");
+      writer.append("      ctx.contentType(\"text/plain\").result(%s);", resultVariableName);
     } else {
-      writer.append("      ctx.contentType(\"%s\").result(result);", produces);
+      writer.append("      ctx.contentType(\"%s\").result(%s);", produces, resultVariableName);
     }
   }
 }

http-generator-javalin/src/main/java/io/avaje/http/generator/javalin/ControllerWriter.java

@@ -87,7 +87,18 @@ private void writeClassStart() {
       writer.append("  private final Validator validator;").eol();
     }
 
-    for (final UType type : jsonTypes.values()) {
+    for (UType type : jsonTypes.values()) {
+      // Support for CompletableFuture's.
+      if (type.mainType().equals("java.util.concurrent.CompletableFuture")) {
+        type = type.paramRaw();
+
+        if (this.jsonTypes.containsKey(type.full())) {
+          // Already written before -- we can skip.
+          continue;
+        }
+      }
+
+      // Everything else
       final var typeString = PrimitiveUtil.wrap(type.shortType()).replace(",", ", ");
       writer.append("  private final JsonType<%s> %sJsonType;", typeString, type.shortName()).eol();
     }
@@ -107,6 +118,12 @@ private void writeClassStart() {
     }
     if (useJsonB) {
       for (final UType type : jsonTypes.values()) {
+        // Skip trying to assign a global variable value for any UType that is a Completable Future. Because the paramRaw() should
+        // already be in this jsonTypes map anyway and write the assignment all by itself.
+        if (type.mainType().equals("java.util.concurrent.CompletableFuture")) {
+          continue;
+        }
+
         JsonBUtil.writeJsonbType(type, writer);
       }
     }

tests/test-javalin-jsonb/src/main/java/org/example/myapp/web/HelloController.java

@@ -5,6 +5,8 @@
 import java.time.LocalDate;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.Executors;
 
 import javax.validation.Valid;
 
@@ -142,6 +144,21 @@ List<HelloDto> getAll() {
     return myService.findAll();
   }
 
+  @Get("/async")
+  CompletableFuture<List<HelloDto>> getAllAsync() {
+    return CompletableFuture.supplyAsync(() -> {
+      // Simulate a delay as if an actual IO operation is being executed.
+      // This also helps ensure that we aren't just getting lucky with timings.
+      try {
+        Thread.sleep(10L);
+      } catch (InterruptedException e) {
+        throw new RuntimeException(e);
+      }
+
+      return myService.findAll();
+    }, Executors.newSingleThreadExecutor()); // Example of how to use a custom executor.
+  }
+
   //  @Hidden
   @Delete(":id")
   void deleteById(int id) {

tests/test-javalin-jsonb/src/test/java/org/example/myapp/HelloControllerTest.java

@@ -62,6 +62,25 @@ void hello2() {
     assertThat(helloDtos).hasSize(2);
   }
 
+  @Test
+  void helloAsyncRequestHandling() {
+    TypeRef<List<HelloDto>> listDto = new TypeRef<List<HelloDto>>() { };
+    final List<HelloDto> beans = given()
+      .get(baseUrl + "/hello/async")
+      .then()
+      .statusCode(200)
+      .extract()
+      .as(listDto);
+
+    assertThat(beans).hasSize(2);
+
+    final List<HelloDto> helloDtos = client.request()
+                                           .path("hello/async")
+                                           .GET().list(HelloDto.class);
+
+    assertThat(helloDtos).hasSize(2);
+  }
+
   @Test
   void getWithPathParamAndQueryParam() {
 

tests/test-javalin/src/main/java/org/example/myapp/web/HelloController.java

@@ -5,6 +5,8 @@
 import java.time.LocalDate;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.Executors;
 
 import javax.validation.Valid;
 
@@ -142,6 +144,21 @@ List<HelloDto> getAll() {
     return myService.findAll();
   }
 
+  @Get("/async")
+  CompletableFuture<List<HelloDto>> getAllAsync() {
+    return CompletableFuture.supplyAsync(() -> {
+      // Simulate a delay as if an actual IO operation is being executed.
+      // This also helps ensure that we aren't just getting lucky with timings.
+      try {
+        Thread.sleep(10L);
+      } catch (InterruptedException e) {
+        throw new RuntimeException(e);
+      }
+
+      return myService.findAll();
+    }, Executors.newSingleThreadExecutor()); // Example of how to use a custom executor.
+  }
+
   //  @Hidden
   @Delete("{id}")
   void deleteById(int id) {
@@ -159,4 +176,5 @@ String getWithMatrixParam(int year, String author, String country, String other,
   String slashAccepting(String name, String nam0, String nam1) {
     return "got name:" + name + " splat0:" + nam0 + " splat1:" + nam1;
   }
+
 }

tests/test-javalin/src/test/java/org/example/myapp/HelloControllerTest.java

@@ -62,6 +62,25 @@ void hello2() {
     assertThat(helloDtos).hasSize(2);
   }
 
+  @Test
+  void helloAsyncRequestHandling() {
+    TypeRef<List<HelloDto>> listDto = new TypeRef<List<HelloDto>>() { };
+    final List<HelloDto> beans = given()
+      .get(baseUrl + "/hello/async")
+      .then()
+      .statusCode(200)
+      .extract()
+      .as(listDto);
+
+    assertThat(beans).hasSize(2);
+
+    final List<HelloDto> helloDtos = client.request()
+      .path("hello/async")
+      .GET().list(HelloDto.class);
+
+    assertThat(helloDtos).hasSize(2);
+  }
+
   @Test
   void getWithPathParamAndQueryParam() {