test(transport): add tests for utf-8 handling in stdio transport

Add test cases to verify proper UTF-8 character handling in StdioClientTransport when system default charset is not UTF-8. Includes a test echo server subprocess to simulate non-UTF-8 environment.

Author: codeboyzhou

mcp-test/src/test/java/io/modelcontextprotocol/client/transport/StdioClientTransportTests.java

@@ -0,0 +1,128 @@
+/*
+ * Copyright 2024-2024 the original author or authors.
+ */
+
+package io.modelcontextprotocol.client.transport;
+
+import io.modelcontextprotocol.json.McpJsonDefaults;
+import io.modelcontextprotocol.spec.McpSchema;
+import org.jspecify.annotations.NonNull;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.Timeout;
+import reactor.test.StepVerifier;
+
+import java.io.BufferedReader;
+import java.io.InputStreamReader;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.FileSystems;
+import java.util.Map;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicReference;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link StdioClientTransport}.
+ *
+ * @author Christian Tzolov
+ */
+@Timeout(30)
+class StdioClientTransportTests {
+
+	static final String FILE_SEPARATOR = FileSystems.getDefault().getSeparator();
+
+	@Test
+	void shouldHandleUtf8MessagesWithNonUtf8DefaultCharset() throws Exception {
+		String utf8Content = "한글 漢字 café 🎉";
+
+		String javaHome = System.getProperty("java.home");
+		String classpath = System.getProperty("java.class.path");
+		String javaExecutable = javaHome + FILE_SEPARATOR + "bin" + FILE_SEPARATOR + "java";
+
+		ServerParameters params = ServerParameters.builder(javaExecutable)
+			.args("-Dfile.encoding=ISO-8859-1", "-cp", classpath, StdioUtf8TestEchoServer.class.getName())
+			.build();
+
+		StdioClientTransport transport = new StdioClientTransport(params, McpJsonDefaults.getMapper());
+
+		AtomicReference<McpSchema.JSONRPCMessage> receivedMessage = new AtomicReference<>();
+		CountDownLatch messageLatch = new CountDownLatch(1);
+
+		StepVerifier.create(transport.connect(message -> {
+			return message.doOnNext(msg -> {
+				receivedMessage.set(msg);
+				messageLatch.countDown();
+			});
+		})).verifyComplete();
+
+		McpSchema.JSONRPCRequest request = new McpSchema.JSONRPCRequest(McpSchema.JSONRPC_VERSION, "echo", 1,
+				Map.of("message", utf8Content));
+
+		StepVerifier.create(transport.sendMessage(request)).verifyComplete();
+
+		assertThat(messageLatch.await(10, TimeUnit.SECONDS)).isTrue();
+
+		assertThat(receivedMessage.get()).isNotNull();
+		assertThat(receivedMessage.get()).isInstanceOf(McpSchema.JSONRPCResponse.class);
+		McpSchema.JSONRPCResponse response = (McpSchema.JSONRPCResponse) receivedMessage.get();
+		assertThat(response.result()).isEqualTo(utf8Content);
+
+		transport.closeGracefully().block();
+	}
+
+	@Test
+	void shouldHandleUtf8ErrorMessagesWithNonUtf8DefaultCharset() throws Exception {
+		String utf8ErrorContent = "错误: 한글 漢字 🎉";
+
+		String javaHome = System.getProperty("java.home");
+		String classpath = System.getProperty("java.class.path");
+		String javaExecutable = javaHome + FILE_SEPARATOR + "bin" + FILE_SEPARATOR + "java";
+
+		ProcessBuilder pb = new ProcessBuilder(javaExecutable, "-Dfile.encoding=ISO-8859-1", "-cp", classpath,
+				StdioUtf8TestEchoServer.class.getName());
+		pb.redirectErrorStream(false);
+
+		Process process = pb.start();
+
+		try {
+			process.getOutputStream()
+				.write(("{\"jsonrpc\":\"2.0\",\"method\":\"echo\",\"params\":{\"message\":\"test\"},\"id\":1}\n")
+					.getBytes(StandardCharsets.UTF_8));
+			process.getOutputStream().flush();
+
+			Thread errorThread = getErrorThread(process, utf8ErrorContent);
+
+			process.waitFor(10, TimeUnit.SECONDS);
+			errorThread.join(1000);
+		}
+		finally {
+			process.destroyForcibly();
+			process.waitFor(10, TimeUnit.SECONDS);
+		}
+	}
+
+	private static @NonNull Thread getErrorThread(Process process, String utf8ErrorContent) {
+		AtomicReference<String> errorContent = new AtomicReference<>();
+		CountDownLatch errorLatch = new CountDownLatch(1);
+
+		Thread errorThread = new Thread(() -> {
+			try (BufferedReader errorReader = new BufferedReader(
+					new InputStreamReader(process.getErrorStream(), StandardCharsets.UTF_8))) {
+				String line;
+				while ((line = errorReader.readLine()) != null) {
+					if (line.contains(utf8ErrorContent)) {
+						errorContent.set(line);
+						errorLatch.countDown();
+						break;
+					}
+				}
+			}
+			catch (Exception ignored) {
+			}
+		});
+		errorThread.start();
+		return errorThread;
+	}
+
+}

mcp-test/src/test/java/io/modelcontextprotocol/client/transport/StdioUtf8TestEchoServer.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright 2024-2024 the original author or authors.
+ */
+
+package io.modelcontextprotocol.client.transport;
+
+import java.io.BufferedReader;
+import java.io.InputStreamReader;
+import java.nio.charset.StandardCharsets;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * Minimal STDIO echo server for testing UTF-8 encoding behavior in StdioClientTransport.
+ *
+ * <p>
+ * This class is spawned as a subprocess with {@code -Dfile.encoding=ISO-8859-1} to
+ * simulate a non-UTF-8 default charset environment. It reads JSON-RPC messages from stdin
+ * and echoes the {@code params.message} value back to stdout, allowing the parent test to
+ * verify that multi-byte UTF-8 characters are preserved.
+ *
+ * @see StdioClientTransportTests#shouldHandleUtf8MessagesWithNonUtf8DefaultCharset
+ */
+public class StdioUtf8TestEchoServer {
+
+	public static void main(String[] args) throws Exception {
+		CountDownLatch latch = new CountDownLatch(1);
+		StringBuilder receivedMessage = new StringBuilder();
+
+		try (BufferedReader reader = new BufferedReader(new InputStreamReader(System.in, StandardCharsets.UTF_8))) {
+			String line;
+			while ((line = reader.readLine()) != null) {
+				if (line.contains("\"echo\"")) {
+					int start = line.indexOf("\"message\":\"") + "\"message\":\"".length();
+					int end = line.indexOf("\"", start);
+					if (start > 0 && end > start) {
+						receivedMessage.append(line, start, end);
+					}
+					String response = "{\"jsonrpc\":\"2.0\",\"id\":1,\"result\":\"" + receivedMessage + "\"}\n";
+					System.out.write(response.getBytes(StandardCharsets.UTF_8));
+					System.out.flush();
+					latch.countDown();
+					break;
+				}
+			}
+		}
+
+		latch.await(5, TimeUnit.SECONDS);
+	}
+
+}