Gradle Kotlin DSL配置为何无法在Java项目中启用System.in？

问题重现 #

运行使用Scanner读取控制台输入的Java程序时，抛出如下异常：

Exception in thread "main" java.util.NoSuchElementException: No line found
    at java.base/java.util.Scanner.nextLine(Scanner.java:1651)
    at App.main(App.java:11)

尝试改用BufferedReader、调整代码逻辑均无效，无论是VSCode Gradle扩展还是Windows命令行执行gradle run都出现相同问题。相关代码与配置如下：

代码 #

import java.util.Scanner;

public class App {
    public static void main(String[] args) {
        // creates an object of Scanner
        Scanner input = new Scanner(System.in);

        System.out.print("Enter your name: ");

        // takes input from the keyboard
        String name = input.nextLine();

        // prints the name
        System.out.println("My name is " + name);
    }
}

原build.gradle.kts配置 #

/*
 * This file was generated by the Gradle 'init' task.
 *
 * This generated file contains a sample Java application project to get you started.
 * For more details on building Java & JVM projects, please refer to https://docs.gradle.org/8.2.1/userguide/building_java_projects.html in the Gradle documentation.
 */

repositories {
    // Use Maven Central for resolving dependencies.
    mavenCentral()
}

dependencies {
    // Use JUnit Jupiter for testing.
    testImplementation("org.junit.jupiter:junit-jupiter:5.9.2")

    testRuntimeOnly("org.junit.platform:junit-platform-launcher")

    // This dependency is used by the application.
    implementation("com.google.guava:guava:31.1-jre")
}

// Apply a specific Java toolchain to ease working on different environments.
java {
    toolchain {
        languageVersion.set(JavaLanguageVersion.of(17))
    }
}

application {
    // Define the main class for the application.
    mainClass.set("project.App")
}

tasks.named<Test>("test") {
    // Use JUnit Platform for unit tests.
    useJUnitPlatform()
}

plugins {
    application
}

tasks.named<JavaExec>("run") {
    standardInput = System.`in`
}

核心原因 #

Gradle的run任务默认未将终端的标准输入（System.in）传递给Java程序，且原配置中plugins块位置靠后，导致显式配置的标准输入关联逻辑未生效。

解决方案 #

1. 调整Gradle配置文件结构 #

将plugins块移至build.gradle.kts最顶部，确保application插件先加载，后续的run任务配置才能生效：

plugins {
    application
}

/* 原注释和其他配置保持不变 */
repositories {
    mavenCentral()
}

dependencies {
    testImplementation("org.junit.jupiter:junit-jupiter:5.9.2")
    testRuntimeOnly("org.junit.platform:junit-platform-launcher")
    implementation("com.google.guava:guava:31.1-jre")
}

java {
    toolchain {
        languageVersion.set(JavaLanguageVersion.of(17))
    }
}

application {
    mainClass.set("project.App")
}

tasks.named<Test>("test") {
    useJUnitPlatform()
}

tasks.named<JavaExec>("run") {
    standardInput = System.`in`
}

2. 确认包结构与主类配置匹配 #

若mainClass.set("project.App")，则App.java必须放在src/main/java/project/目录下，保证Gradle能正确定位主类。

3. 使用交互式终端运行 #

• 在VSCode中，打开集成终端直接执行gradle run，而非通过扩展的"Run"按钮；
• 命令行可添加参数确保输出正常：

  gradle run --console=plain
  

4. 代码中添加输入校验（可选） #

为避免异常，可在读取前检查输入是否存在：

import java.util.Scanner;

public class App {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        System.out.print("Enter your name: ");
        
        if (input.hasNextLine()) {
            String name = input.nextLine();
            System.out.println("My name is " + name);
        } else {
            System.out.println("未获取到输入");
        }
        
        input.close();
    }
}

内容的提问来源于stack exchange，提问作者Confused