fix(#6): NullPointerException for tools that return void

Author: codeboyzhou

src/main/java/com/github/codeboyzhou/mcp/declarative/server/component/McpServerTool.java

@@ -93,7 +93,8 @@ public McpSchema.CallToolResult invoke(
       result = e + ": " + e.getMessage();
       isError = true;
     }
-    McpSchema.Content content = new McpSchema.TextContent(result.toString());
+    final String text = result == null ? "This tool returned nullable or void" : result.toString();
+    McpSchema.Content content = new McpSchema.TextContent(text);
     return new McpSchema.CallToolResult(List.of(content), isError);
   }
 

src/test/java/com/github/codeboyzhou/mcp/declarative/McpServersTest.java

@@ -3,6 +3,7 @@
 import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertSame;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
@@ -184,19 +185,43 @@ private void verifyPromptsRegistered(McpSyncClient client) {
     List<McpSchema.Prompt> prompts = client.listPrompts().prompts();
     assertEquals(2, prompts.size());
 
-    McpSchema.Prompt prompt = prompts.get(0);
-    assertEquals("prompt1_name", prompt.name());
-    assertEquals("prompt1_title", prompt.title());
-    assertEquals("prompt1_description", prompt.description());
-
-    List<McpSchema.PromptArgument> arguments = prompt.arguments();
-    assertEquals(2, arguments.size());
-    assertEquals("param1", arguments.get(0).name());
-    assertEquals("param1_title", arguments.get(0).title());
-    assertEquals("param1_description", arguments.get(0).description());
-    assertEquals("param2", arguments.get(1).name());
-    assertEquals("param2_title", arguments.get(1).title());
-    assertEquals("param2_description", arguments.get(1).description());
+    McpSchema.Prompt prompt1 =
+        prompts.stream()
+            .filter(prompt -> prompt.name().equals("prompt1_name"))
+            .findAny()
+            .orElse(null);
+    assertNotNull(prompt1);
+    assertEquals("prompt1_name", prompt1.name());
+    assertEquals("prompt1_title", prompt1.title());
+    assertEquals("prompt1_description", prompt1.description());
+
+    List<McpSchema.PromptArgument> arguments1 = prompt1.arguments();
+    assertEquals(2, arguments1.size());
+    assertEquals("param1", arguments1.get(0).name());
+    assertEquals("param1_title", arguments1.get(0).title());
+    assertEquals("param1_description", arguments1.get(0).description());
+    assertEquals("param2", arguments1.get(1).name());
+    assertEquals("param2_title", arguments1.get(1).title());
+    assertEquals("param2_description", arguments1.get(1).description());
+
+    McpSchema.Prompt prompt2 =
+        prompts.stream()
+            .filter(prompt -> prompt.name().equals("prompt2_name"))
+            .findAny()
+            .orElse(null);
+    assertNotNull(prompt2);
+    assertEquals("prompt2_name", prompt2.name());
+    assertEquals("prompt2_title", prompt2.title());
+    assertEquals("prompt2_description", prompt2.description());
+
+    List<McpSchema.PromptArgument> arguments2 = prompt2.arguments();
+    assertEquals(2, arguments2.size());
+    assertEquals("param1", arguments2.get(0).name());
+    assertEquals("param1_title", arguments2.get(0).title());
+    assertEquals("param1_description", arguments2.get(0).description());
+    assertEquals("param2", arguments2.get(1).name());
+    assertEquals("param2_title", arguments2.get(1).title());
+    assertEquals("param2_description", arguments2.get(1).description());
   }
 
   private void verifyPromptsCalled(McpSyncClient client) {
@@ -219,17 +244,28 @@ private void verifyPromptsCalled(McpSyncClient client) {
 
   private void verifyToolsRegistered(McpSyncClient client) {
     List<McpSchema.Tool> tools = client.listTools().tools();
-    assertEquals(2, tools.size());
+    assertEquals(3, tools.size());
 
-    McpSchema.Tool tool1 = tools.get(0);
+    McpSchema.Tool tool1 =
+        tools.stream().filter(tool -> tool.name().equals("tool1_name")).findAny().orElse(null);
+    assertNotNull(tool1);
     assertEquals("tool1_name", tool1.name());
     assertEquals("tool1_title", tool1.title());
     assertEquals("tool1_description", tool1.description());
 
-    McpSchema.Tool tool2 = tools.get(1);
+    McpSchema.Tool tool2 =
+        tools.stream().filter(tool -> tool.name().equals("tool2_name")).findAny().orElse(null);
+    assertNotNull(tool2);
     assertEquals("tool2_name", tool2.name());
     assertEquals("tool2_title", tool2.title());
     assertEquals("tool2_description", tool2.description());
+
+    McpSchema.Tool tool3 =
+        tools.stream().filter(tool -> tool.name().equals("tool3_name")).findAny().orElse(null);
+    assertNotNull(tool3);
+    assertEquals("tool3_name", tool3.name());
+    assertEquals("tool3_title", tool3.title());
+    assertEquals("tool3_description", tool3.description());
   }
 
   private void verifyToolsCalled(McpSyncClient client) {
@@ -248,5 +284,13 @@ private void verifyToolsCalled(McpSyncClient client) {
     McpSchema.TextContent content2 = (McpSchema.TextContent) result2.content().get(0);
     assertFalse(result2.isError());
     assertEquals("tool2 is called", content2.text());
+
+    String name3 = "tool3_name";
+    Map<String, Object> args3 = Map.of("param1", "value1", "param2", "value2");
+    McpSchema.CallToolRequest request3 = new McpSchema.CallToolRequest(name3, args3);
+    McpSchema.CallToolResult result3 = client.callTool(request3);
+    McpSchema.TextContent content3 = (McpSchema.TextContent) result3.content().get(0);
+    assertFalse(result3.isError());
+    assertEquals("This tool returned nullable or void", content3.text());
   }
 }

src/test/java/com/github/codeboyzhou/mcp/declarative/test/TestMcpPrompts.java

@@ -16,6 +16,7 @@ public String prompt1(
       @McpPromptParam(name = "param2", title = "param2_title", description = "param2_description")
           String param2,
       String param3) {
+
     log.debug("prompt1 called with params: {}, {}, {}", param1, param2, param3);
     return "prompt1 is called";
   }
@@ -27,6 +28,7 @@ public String prompt2(
       @McpPromptParam(name = "param2", title = "param2_title", description = "param2_description")
           String param2,
       String param3) {
+
     log.debug("prompt2 called with params: {}, {}, {}", param1, param2, param3);
     return "prompt2 is called";
   }

src/test/java/com/github/codeboyzhou/mcp/declarative/test/TestMcpTools.java

@@ -11,21 +11,30 @@ public class TestMcpTools {
 
   @McpTool(name = "tool1_name", title = "tool1_title", description = "tool1_description")
   public String tool1(
-      @McpToolParam(name = "param1", description = "param1_description") String param1,
-      @McpToolParam(name = "param2", description = "param2_description", required = true)
-          String param2,
-      String param3) {
-    log.debug("tool1 called with params: {}, {}, {}", param1, param2, param3);
+      @McpToolParam(name = "param1", description = "param1_description") String p1,
+      @McpToolParam(name = "param2", description = "param2_description", required = true) String p2,
+      String p3) {
+
+    log.debug("tool1 called with params: {}, {}, {}", p1, p2, p3);
     return "tool1 is called";
   }
 
   @McpTool(name = "tool2_name", title = "tool2_title", description = "tool2_description")
   public String tool2(
-      @McpToolParam(name = "param1", description = "param1_description") String param1,
-      @McpToolParam(name = "param2", description = "param2_description", required = true)
-          String param2,
-      String param3) {
-    log.debug("tool2 called with params: {}, {}, {}", param1, param2, param3);
+      @McpToolParam(name = "param1", description = "param1_description") String p1,
+      @McpToolParam(name = "param2", description = "param2_description", required = true) String p2,
+      String p3) {
+
+    log.debug("tool2 called with params: {}, {}, {}", p1, p2, p3);
     return "tool2 is called";
   }
+
+  @McpTool(name = "tool3_name", title = "tool3_title", description = "tool3_description")
+  public void tool3(
+      @McpToolParam(name = "param1", description = "param1_description") String p1,
+      @McpToolParam(name = "param2", description = "param2_description", required = true) String p2,
+      String p3) {
+
+    log.debug("tool3 called with params: {}, {}, {}, and no return value", p1, p2, p3);
+  }
 }