Upgrade to Spring Boot 4.0

Author: bclozel

README.adoc

@@ -1,4 +1,4 @@
-:RestTemplate: http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/web/client/RestTemplate.html
+:RestClient: https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/web/client/RestClient.html
 :toc:
 :icons: font
 :source-highlighter: prettify
@@ -21,14 +21,14 @@ include::https://raw.githubusercontent.com/spring-guides/getting-started-macros/
 [[scratch]]
 == Starting with Spring Initializr
 
-You can use this https://start.spring.io/#!type=maven-project&groupId=com.example&artifactId=consuming-rest&name=consuming-rest&description=Demo%20project%20for%20Spring%20Boot&packageName=com.example.consuming-rest&dependencies=web[pre-initialized project] and click Generate to download a ZIP file. This project is configured to fit the examples in this tutorial.
+You can use this https://start.spring.io/#!type=gradle-project&language=java&packaging=jar&groupId=com.example&artifactId=consuming-rest&name=consuming-rest&packageName=com.example.consuming-rest&dependencies=spring-restclient[pre-initialized project] and click Generate to download a ZIP file. This project is configured to fit the examples in this tutorial.
 
 To manually initialize the project:
 
 . Navigate to https://start.spring.io.
 This service pulls in all the dependencies you need for an application and does most of the setup for you.
 . Choose either Gradle or Maven and the language you want to use.
-. Click *Dependencies* and select *Spring Web*.
+. Click *Dependencies* and select *HTTP Client*.
 . Click *Generate*.
 . Download the resulting ZIP file, which is an archive of a web application that is configured with your choices.
 
@@ -69,8 +69,8 @@ that looks something like this:
 That is easy enough but not terribly useful when fetched through a browser or through curl.
 
 A more useful way to consume a REST web service is programmatically. To help you with that
-task, Spring provides a convenient template class called {RestTemplate}[`RestTemplate`].
-`RestTemplate` makes interacting with most RESTful services a one-line incantation. And it
+task, Spring provides a convenient template class called {RestClient}[`RestClient`].
+`RestClient` makes interacting with most RESTful services a one-line incantation. And it
 can even bind that data to custom domain types.
 
 First, you need to create a domain class, a Java record class or Kotlin data class, to contain the data that you need.
@@ -135,10 +135,8 @@ Now you need to add a few other things to the `ConsumingRestApplication` class t
 show quotations from our RESTful source. You need to add:
 
 * A logger, to send output to the log (the console, in this example).
-* A `RestTemplate`, which uses the Jackson JSON processing library to process the incoming
-data.
-* A `CommandLineRunner` that runs the `RestTemplate` (and, consequently, fetches our
-quotation) on startup.
+* A `CommandLineRunner` that builds a `RestClient` instance with the auto-configured `RestClient.Builder`
+ and uses it to fetch our quotation on startup.
 
 The following listing shows the finished `ConsumingRestApplication` class:
 
@@ -155,14 +153,6 @@ include::complete-kotlin/src/main/kotlin/com/example/consumingrest/ConsumingRest
 ----
 ====
 
-Finally, you need to set the server port. The quoters application uses the default 
-server port, 8080, so this application cannot also use the same port. You can set 
-the server port to 8081 by adding the following line to application properties 
-(which the Initializr created for you):
-
-----
-server.port=8081
-----
 
 == Running the Application
 

complete-kotlin/build.gradle.kts

@@ -1,27 +1,35 @@
 plugins {
-    kotlin("jvm") version "1.9.25"
-    kotlin("plugin.spring") version "1.9.25"
-    id("org.springframework.boot") version "3.5.6"
+    kotlin("jvm") version "2.2.21"
+    kotlin("plugin.spring") version "2.2.21"
+    id("org.springframework.boot") version "4.0.0"
     id("io.spring.dependency-management") version "1.1.7"
 }
 
 group = "com.example"
 version = "0.0.1-SNAPSHOT"
-java.sourceCompatibility = JavaVersion.VERSION_17
+
+java {
+    toolchain {
+        languageVersion = JavaLanguageVersion.of(17)
+    }
+}
 
 repositories {
     mavenCentral()
 }
 
 dependencies {
-    implementation("org.springframework.boot:spring-boot-starter-web")
-    testImplementation("org.springframework.boot:spring-boot-starter-test")
+    implementation("org.springframework.boot:spring-boot-starter-restclient")
+    implementation("com.fasterxml.jackson.module:jackson-module-kotlin")
+    implementation("org.jetbrains.kotlin:kotlin-reflect")
+    testImplementation("org.springframework.boot:spring-boot-starter-restclient-test")
+    testImplementation("org.jetbrains.kotlin:kotlin-test-junit5")
+    testRuntimeOnly("org.junit.platform:junit-platform-launcher")
 }
 
 kotlin {
-    jvmToolchain(17)
     compilerOptions {
-        freeCompilerArgs.addAll("-Xjsr305=strict")
+        freeCompilerArgs.addAll("-Xjsr305=strict", "-Xannotation-default-target=param-property")
     }
 }
 

complete-kotlin/gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-8.14.3-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-9.2.1-bin.zip
 networkTimeout=10000
 validateDistributionUrl=true
 zipStoreBase=GRADLE_USER_HOME

complete-kotlin/src/main/kotlin/com/example/consumingrest/ConsumingRestApplication.kt

@@ -1,29 +1,24 @@
 package com.example.consumingrest
 
 import org.slf4j.LoggerFactory
-import org.springframework.boot.CommandLineRunner
+import org.springframework.boot.ApplicationRunner
 import org.springframework.boot.autoconfigure.SpringBootApplication
 import org.springframework.boot.runApplication
-import org.springframework.boot.web.client.RestTemplateBuilder
 import org.springframework.context.annotation.Bean
 import org.springframework.context.annotation.Profile
-import org.springframework.web.client.RestTemplate
-import org.springframework.web.client.getForObject
+import org.springframework.web.client.RestClient
+import org.springframework.web.client.toEntity
 
 private val log = LoggerFactory.getLogger(ConsumingRestApplication::class.java)
 
 @SpringBootApplication
 class ConsumingRestApplication {
 
-    @Bean
-    fun restTemplate(builder: RestTemplateBuilder): RestTemplate {
-        return builder.build()
-    }
-
     @Bean
     @Profile("!test")
-    fun run(restTemplate: RestTemplate) = CommandLineRunner {
-        val quote = restTemplate.getForObject<Quote>("http://localhost:8080/api/random")
+    fun run(builder: RestClient.Builder) = ApplicationRunner {
+        val quote = builder.build().get().uri("http://localhost:8080/api/random")
+            .retrieve().toEntity<Quote>()
         log.info(quote.toString())
     }
 }

complete-kotlin/src/test/kotlin/com/example/consumingrest/ConsumingRestApplicationTests.kt

@@ -16,22 +16,15 @@
 
 package com.example.consumingrest
 
-import org.assertj.core.api.Assertions.assertThat
 import org.junit.jupiter.api.Test
-import org.springframework.beans.factory.annotation.Autowired
 import org.springframework.boot.test.context.SpringBootTest
 import org.springframework.test.context.ActiveProfiles
-import org.springframework.web.client.RestTemplate
 
 @SpringBootTest
 @ActiveProfiles("test")
 class ConsumingRestApplicationTest {
 
-    @Autowired
-    private lateinit var restTemplate: RestTemplate
-
     @Test
     fun contextLoads() {
-        assertThat(restTemplate).isNotNull
     }
 }

complete/build.gradle

@@ -1,20 +1,25 @@
 plugins {
-	id 'org.springframework.boot' version '3.5.6'
+	id 'org.springframework.boot' version '4.0.0'
 	id 'io.spring.dependency-management' version '1.1.7'
 	id 'java'
 }
 
 group = 'com.example'
 version = '0.0.1-SNAPSHOT'
-sourceCompatibility = '17'
+
+java {
+	toolchain {
+		languageVersion = JavaLanguageVersion.of(17)
+	}
+}
 
 repositories {
 	mavenCentral()
 }
 
 dependencies {
-	implementation 'org.springframework.boot:spring-boot-starter-web'
-	testImplementation 'org.springframework.boot:spring-boot-starter-test'
+	implementation 'org.springframework.boot:spring-boot-starter-restclient'
+	testImplementation 'org.springframework.boot:spring-boot-starter-restclient-test'
 }
 
 test {

complete/gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-8.14.3-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-9.2.1-bin.zip
 networkTimeout=10000
 validateDistributionUrl=true
 zipStoreBase=GRADLE_USER_HOME

complete/pom.xml

@@ -5,7 +5,7 @@
 	<parent>
 		<groupId>org.springframework.boot</groupId>
 		<artifactId>spring-boot-starter-parent</artifactId>
-		<version>3.5.6</version>
+		<version>4.0.0</version>
 		<relativePath/> <!-- lookup parent from repository -->
 	</parent>
 	<groupId>com.example</groupId>
@@ -21,12 +21,12 @@
 	<dependencies>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
-			<artifactId>spring-boot-starter-web</artifactId>
+			<artifactId>spring-boot-starter-restclient</artifactId>
 		</dependency>
 
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
-			<artifactId>spring-boot-starter-test</artifactId>
+			<artifactId>spring-boot-starter-restclient-test</artifactId>
 			<scope>test</scope>
 		</dependency>
 	</dependencies>

complete/src/main/java/com/example/consumingrest/ConsumingRestApplication.java

@@ -2,13 +2,13 @@
 
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
-import org.springframework.boot.CommandLineRunner;
+
+import org.springframework.boot.ApplicationRunner;
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.autoconfigure.SpringBootApplication;
-import org.springframework.boot.web.client.RestTemplateBuilder;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Profile;
-import org.springframework.web.client.RestTemplate;
+import org.springframework.web.client.RestClient;
 
 @SpringBootApplication
 public class ConsumingRestApplication {
@@ -19,17 +19,15 @@ public static void main(String[] args) {
 		SpringApplication.run(ConsumingRestApplication.class, args);
 	}
 
-	@Bean
-	public RestTemplate restTemplate(RestTemplateBuilder builder) {
-		return builder.build();
-	}
-
 	@Bean
 	@Profile("!test")
-	public CommandLineRunner run(RestTemplate restTemplate) throws Exception {
+	public ApplicationRunner run(RestClient.Builder builder) {
+		RestClient restClient = builder.baseUrl("http://localhost:8080").build();
 		return args -> {
-			Quote quote = restTemplate.getForObject(
-					"http://localhost:8080/api/random", Quote.class);
+			Quote quote = restClient
+					.get().uri("/api/random")
+					.retrieve()
+					.body(Quote.class);
 			log.info(quote.toString());
 		};
 	}

complete/src/test/java/com/example/consumingrest/ConsumingRestApplicationTest.java

@@ -16,25 +16,17 @@
 
 package com.example.consumingrest;
 
-import static org.assertj.core.api.Assertions.assertThat;
-
 import org.junit.jupiter.api.Test;
 
-import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.test.context.SpringBootTest;
 import org.springframework.test.context.ActiveProfiles;
-import org.springframework.web.client.RestTemplate;
 
 @SpringBootTest
 @ActiveProfiles("test")
 public class ConsumingRestApplicationTest {
 
-	@Autowired
-	private RestTemplate restTemplate;
-
 	@Test
 	public void contextLoads() {
-		assertThat(restTemplate).isNotNull();
 	}
 
 }