问题描述
当我发现它不断抛出java.util.NoSuchElementException: No line found
时,我正在研究一个涉及读取另一个进程的项目,因此我制作了一个Maven测试应用程序来检查它是Heroku的错而不是程序的错。
src/main/java/com/test/App.java
:
package com.test;
import java.util.Arrays;
import java.util.Scanner;
import java.io.IOException;
public class App{
public static void main(String[] args) throws IOException{
System.out.println(new Scanner(new ProcessBuilder(Arrays.asList("java","A")).start().getInputStream()).nextLine());
}
}
A.java
:
public class A{
public static void main(String[] args){
System.out.println("Test");
}
}
pom.xml
:
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.test</groupId>
<artifactId>test</artifactId>
<version>1.0</version>
<name>test</name>
<!-- FIXME change it to the project's website -->
<url>http://www.example.com</url>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<maven.compiler.source>1.7</maven.compiler.source>
<maven.compiler.target>1.7</maven.compiler.target>
</properties>
<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.11</version>
<scope>test</scope>
</dependency>
</dependencies>
<build>
<pluginManagement><!-- lock down plugins versions to avoid using Maven defaults (may be moved to parent pom) -->
<plugins>
<!-- clean lifecycle,see https://maven.apache.org/ref/current/maven-core/lifecycles.html#clean_Lifecycle -->
<plugin>
<artifactId>maven-clean-plugin</artifactId>
<version>3.1.0</version>
</plugin>
<!-- default lifecycle,jar packaging: see https://maven.apache.org/ref/current/maven-core/default-bindings.html#Plugin_bindings_for_jar_packaging -->
<plugin>
<artifactId>maven-resources-plugin</artifactId>
<version>3.0.2</version>
</plugin>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.8.0</version>
</plugin>
<plugin>
<artifactId>maven-surefire-plugin</artifactId>
<version>2.22.1</version>
</plugin>
<plugin>
<artifactId>maven-jar-plugin</artifactId>
<version>3.0.2</version>
<configuration>
<archive>
<manifest>
<mainClass>com.test.App</mainClass>
</manifest>
</archive>
</configuration>
</plugin>
<plugin>
<artifactId>maven-install-plugin</artifactId>
<version>2.5.2</version>
</plugin>
<plugin>
<artifactId>maven-deploy-plugin</artifactId>
<version>2.8.2</version>
</plugin>
<!-- site lifecycle,see https://maven.apache.org/ref/current/maven-core/lifecycles.html#site_Lifecycle -->
<plugin>
<artifactId>maven-site-plugin</artifactId>
<version>3.7.1</version>
</plugin>
<plugin>
<artifactId>maven-project-info-reports-plugin</artifactId>
<version>3.0.0</version>
</plugin>
</plugins>
</pluginManagement>
</build>
</project>
在javac A.java
,mvn install
和java -jar target/test-1.0.jar
之后,它在我的计算机上打印“ Test”,但在Heroku中抛出NoSuchElementException
。
完整的堆栈跟踪:
Exception in thread "main" java.util.NoSuchElementException: No line found
at java.util.Scanner.nextLine(Scanner.java:1540)
at com.test.App.main(App.java:7)
有人知道这是什么原因以及如何解决?
编辑:加入Github repo以期使其更清晰
编辑编辑:奇怪的是,如果我将A.java
放在src/main/java
中并使用@Malax的答案中的命令,它可以工作,但是我仍然不明白为什么Heroku看不到Java文件的原因在src
之外。也许正在修剪它们以节省空间?
解决方法
发生此问题是因为您在nextLine
上呼叫Scanner
而没有检查是否还有一行。您需要检查hasNext
,以确保nextLine
不会失败。
没有任何行的根本原因是调用java
时没有指定类路径。如果没有它,java
将无法找到您编译的A
类,并且会失败,例如
Error: Could not find or load main class A
Caused by: java.lang.ClassNotFoundException: A
在STDERR上,您没有用getInputStream
捕获。改为使用getErrorStream
时,您将能够读取错误消息(但是,您将不会收到STDOUT消息)。
这与Heroku无关,我只能推测为什么它在本地对您有用。要提供java
的类路径,可以使用-cp
:
ProcessBuilder processBuilder = new ProcessBuilder(Arrays.asList("java","-cp","target/classes","A"));
Scanner scanner = new Scanner(processBuilder.start().getInputStream());
while (scanner.hasNext()) {
System.out.println(scanner.nextLine());
}