fix cascading of Map - cascade auto-register the value type of Map property

This assumes the key type of the Map is String in the json sense

Author: rbygrave

blackbox-test/src/main/java/org/example/customer/cascade/MCChild2.java

@@ -0,0 +1,4 @@
+package org.example.customer.cascade;
+
+public record MCChild2(int myValue) {
+}

blackbox-test/src/main/java/org/example/customer/cascade/MCTop.java

@@ -2,8 +2,14 @@
 
 import io.avaje.jsonb.Json;
 
+import java.util.Collections;
 import java.util.List;
+import java.util.Map;
 
 @Json
-public record MCTop(int id, MCOther other, List<MCChild> children) {
+public record MCTop(int id, MCOther other, List<MCChild> children, Map<String,MCChild2> childMap) {
+
+  public MCTop(int id, MCOther foo, List<MCChild> children) {
+    this(id, foo, children, Collections.emptyMap());
+  }
 }

blackbox-test/src/test/java/org/example/customer/cascade/MCTopTest.java

@@ -4,8 +4,7 @@
 import io.avaje.jsonb.Jsonb;
 import org.junit.jupiter.api.Test;
 
-import java.util.Collections;
-import java.util.List;
+import java.util.*;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -18,9 +17,12 @@ void toJson() {
     JsonType<MCTop> type = jsonb.type(MCTop.class);
 
     String asJson = type.toJson(new MCTop(42, new MCOther("foo"), Collections.emptyList()));
-    assertThat(asJson).isEqualTo("{\"id\":42,\"other\":{\"other\":\"foo\"}}");
+    assertThat(asJson).isEqualTo("{\"id\":42,\"other\":{\"other\":\"foo\"},\"childMap\":{}}");
 
     String asJson2 = type.toJson(new MCTop(42, new MCOther("foo"), List.of(new MCChild(23), new MCChild(24))));
-    assertThat(asJson2).isEqualTo("{\"id\":42,\"other\":{\"other\":\"foo\"},\"children\":[{\"myValue\":23},{\"myValue\":24}]}");
+    assertThat(asJson2).isEqualTo("{\"id\":42,\"other\":{\"other\":\"foo\"},\"children\":[{\"myValue\":23},{\"myValue\":24}],\"childMap\":{}}");
+
+    String asJsonWithMap = type.toJson(new MCTop(42, new MCOther("foo"), Collections.emptyList(), new TreeMap<>(Map.of("k0", new MCChild2(98), "k1", new MCChild2(99)))));
+    assertThat(asJsonWithMap).isEqualTo("{\"id\":42,\"other\":{\"other\":\"foo\"},\"childMap\":{\"k0\":{\"myValue\":98},\"k1\":{\"myValue\":99}}}");
   }
 }

jsonb-generator/src/main/java/io/avaje/jsonb/generator/FieldReader.java

@@ -108,6 +108,8 @@ void cascadeTypes(Set<String> types) {
       String topType = genericType.topType();
       if (topType.equals("java.util.List") || topType.equals("java.util.Set")) {
         types.add(genericType.firstParamType());
+      } else if (topType.equals("java.util.Map")) {
+        types.add(genericType.secondParamType());
       } else {
         types.add(topType);
       }

jsonb-generator/src/main/java/io/avaje/jsonb/generator/GenericType.java

@@ -181,9 +181,10 @@ private String asTypeContainer() {
   }
 
   String firstParamType() {
-    if (params.isEmpty()) {
-      return "java.lang.Object";
-    }
-    return params.get(0).topType();
+    return params.isEmpty() ? "java.lang.Object" : params.get(0).topType();
+  }
+
+  String secondParamType() {
+    return params.size() != 2 ? "java.lang.Object" : params.get(1).topType();
   }
 }