Java – 从资源文件夹中读取文件

分手后的思念是犯贱 2023-09-26 09:36 131阅读 0赞

在Java中，我们可以使用getResourceAsStream或getResource从resources文件夹或类路径的根读取一个或多个文件。

format_png

getResourceAsStream方法返回一个InputStream .

  1. // the stream holding the file content
  2.   InputStream is = getClass().getClassLoader().getResourceAsStream("file.txt");
  4.   // for static access, uses the class name directly
  5.   InputStream is = JavaClassName.class.getClassLoader().getResourceAsStream("file.txt");

getResource方法返回 一个URL，通常将其转换为File ;在 JAR 文件中不起作用。

  1. // get the file url, not working in JAR file.
  2.   URL resource = getClass().getClassLoader().getResource("file.txt");
  3.   if (resource == null) {
  4.       throw new IllegalArgumentException("file not found!");
  5.   } else {
  7.       // failed if files have whitespaces or special characters
  8.       //return new File(resource.getFile());
  10.       return new File(resource.toURI());
  11.   }
  13.   // for static access
  14.   // URL resource = JavaClassName.class.getClassLoader().getResource("fileName");

1. 资源文件夹中的文件

1.1 查看src/main/resources中的文件，稍后我们将访问这些文件并打印出文件内容。

b9a57fc5f3274c0eb67e34db22e1bba4.webp

src/main/resources/database.properties

  1. datasource.url=jdbc:mysql://localhost/favtuts?useSSL=false
  2. datasource.username=root
  3. datasource.password=password
  4. datasource.driver-class-name=com.mysql.jdbc.Driver

src/main/resources/json/file1.json

  1. {
  2.   "name": "favtuts",
  3.   "age": 38
  4. }

src/main/resources/json/file2.json

  1. {
  2.   "name": "jack",
  3.   "age": 40
  4. }

src/main/resources/json/sub/subfile1.json

  1. {
  2.   "name": "sub",
  3.   "age": 99
  4. }

1.2 默认情况下，构建工具（如 Maven、Gradle 或常见的 Java 实践）会将所有文件从src/main/resources复制到target/classes或build/classes的根目录。因此，当我们尝试从src/main/resources读取文件时，我们从项目类路径的根读取该文件。

format_png

1.3 下面是一个 JAR 文件结构。通常，resources文件夹中的文件将复制到类路径的根目录。

  1. $ jar -tf target/example.jar
  2. META-INF/MANIFEST.MF
  3. META-INF/
  4. json/
  5. json/sub/
  6. json/file2.json
  7. json/sub/subfile1.json
  8. json/file1.json
  9. database.properties
  10. com/
  11. com/favtuts/
  12. com/favtuts/io/
  13. com/favtuts/io/utils/
  14. //...

2. 从资源文件夹中获取文件。

2.1 下面的示例演示如何使用getResourceAsStream和getResource方法从resources文件夹中读取json/file1.json文件并打印出文件内容。

注意

  • getResource方法在 JAR 文件中不起作用。
  • getResourceAsStream方法在任何地方都有效。

FileResourcesUtils.java

  1. package com.favtuts.io.utils;
  3. import java.io.BufferedReader;
  4. import java.io.File;
  5. import java.io.IOException;
  6. import java.io.InputStream;
  7. import java.io.InputStreamReader;
  8. import java.net.URISyntaxException;
  9. import java.net.URL;
  10. import java.nio.charset.StandardCharsets;
  11. import java.nio.file.Files;
  12. import java.util.List;
  14. public class FileResourcesUtils {
  16.     public static void main(String[] args) throws Exception {
  18.         FileResourcesUtils app = new FileResourcesUtils();
  20.         //String fileName = "database.properties";
  21.         String fileName = "json/file1.json";
  23.         System.out.println("getResourceAsStream : " + fileName);
  24.         InputStream is = app.getFileFromResourceAsStream(fileName);
  25.         printInputStream(is);
  27.         System.out.println("\ngetResource : " + fileName);
  28.         File file = app.getFileFromResource(fileName);
  29.         printFile(file);
  31.     }
  33.     // get a file from the resources folder
  34.     // works everywhere, IDEA, unit test and JAR file.
  35.     private InputStream getFileFromResourceAsStream(String fileName) {
  37.         // The class loader that loaded the class
  38.         ClassLoader classLoader = getClass().getClassLoader();
  39.         InputStream inputStream = classLoader.getResourceAsStream(fileName);
  41.         // the stream holding the file content
  42.         if (inputStream == null) {
  43.             throw new IllegalArgumentException("file not found! " + fileName);
  44.         } else {
  45.             return inputStream;
  46.         }
  48.     }
  50.     /*
  51.         The resource URL is not working in the JAR
  52.         If we try to access a file that is inside a JAR,
  53.         It throws NoSuchFileException (linux), InvalidPathException (Windows)
  55.         Resource URL Sample: file:java-io.jar!/json/file1.json
  56.      */
  57.     private File getFileFromResource(String fileName) throws URISyntaxException{
  59.         ClassLoader classLoader = getClass().getClassLoader();
  60.         URL resource = classLoader.getResource(fileName);
  61.         if (resource == null) {
  62.             throw new IllegalArgumentException("file not found! " + fileName);
  63.         } else {
  65.             // failed if files have whitespaces or special characters
  66.             //return new File(resource.getFile());
  68.             return new File(resource.toURI());
  69.         }
  71.     }
  73.     // print input stream
  74.     private static void printInputStream(InputStream is) {
  76.         try (InputStreamReader streamReader =
  77.                     new InputStreamReader(is, StandardCharsets.UTF_8);
  78.              BufferedReader reader = new BufferedReader(streamReader)) {
  80.             String line;
  81.             while ((line = reader.readLine()) != null) {
  82.                 System.out.println(line);
  83.             }
  85.         } catch (IOException e) {
  86.             e.printStackTrace();
  87.         }
  89.     }
  91.     // print a file
  92.     private static void printFile(File file) {
  94.         List<String> lines;
  95.         try {
  96.             lines = Files.readAllLines(file.toPath(), StandardCharsets.UTF_8);
  97.             lines.forEach(System.out::println);
  98.         } catch (IOException e) {
  99.             e.printStackTrace();
  100.         }
  102.     }
  103. }

输出

  1. getResourceAsStream : json/file1.json
  2. {
  3. "name": "favtuts",
  4. "age": 38
  5. }
  6. getResource : json/file1.json
  7. {
  8. "name": "favtuts",
  9. "age": 38
  10. }

2.2 现在，我们将项目打包到一个JAR文件中并运行它;这一次，getResource将失败并返回NoSuchFileException或InvalidPathException。我们无法通过资源 URL 读取 JAR 文件中的文件。

在 Linux （Ubuntu） 上运行 JAR 文件，它会引发NoSuchFileException。

  1. $ mvn clean package
  2. $ java -jar target/java-io.jar
  3. $ jar tf target/java-io.jar | more
  4. $ java -cp target/java-io.jar com.favtuts.io.utils.FileResourcesUtils
  5. getResourceAsStream : json/file1.json
  6. {
  7. "name": "favtuts",
  8. "age": 38
  9. }
  10. # for new File(resource.getFile());
  11. getResource : json/file1.json
  12. java.nio.file.NoSuchFileException: file:/home/favtuts/projects/core-java/java-io/target/java-io.jar!/json/file1.json
  13. at java.base/sun.nio.fs.UnixException.translateToIOException(UnixException.java:92)
  14. at java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:111)
  15. at java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:116)
  16. at java.base/sun.nio.fs.UnixFileSystemProvider.newByteChannel(UnixFileSystemProvider.java:219)
  17. at java.base/java.nio.file.Files.newByteChannel(Files.java:370)
  18. at java.base/java.nio.file.Files.newByteChannel(Files.java:421)
  19. at java.base/java.nio.file.spi.FileSystemProvider.newInputStream(FileSystemProvider.java:420)
  20. at java.base/java.nio.file.Files.newInputStream(Files.java:155)
  21. at java.base/java.nio.file.Files.newBufferedReader(Files.java:2838)
  22. at java.base/java.nio.file.Files.readAllLines(Files.java:3329)
  23. at com.favtuts.io.utils.FileResourcesUtils.printFile(FileResourcesUtils.java:135)
  24. at com.favtuts.io.utils.FileResourcesUtils.main(FileResourcesUtils.java:24)
  25. # for new File(resource.toURI());
  26. getResource : json/file1.json
  27. Exception in thread "main" java.lang.IllegalArgumentException: URI is not hierarchical
  28. at java.base/java.io.File.<init>(File.java:420)
  29. at com.favtuts.io.utils.FileResourcesUtils.getFileFromResource(FileResourcesUtils.java:112)
  30. at com.favtuts.io.utils.FileResourcesUtils.main(FileResourcesUtils.java:29)

您可能会收到以下消息

  1. no main manifest attribute, in "java-io.jar"

要解决此问题，请阅读堆栈溢出：无法执行 jar 文件：“无主清单属性”中的指南

对于 Maven，我们应该使用在 pom 中定义的 maven-jar 插件.xml：

  1. <!-- Make this jar executable -->
  2.         <plugin>
  3.           <groupId>org.apache.maven.plugins</groupId>
  4.           <artifactId>maven-jar-plugin</artifactId>
  5.           <version>3.2.0</version>
  6.           <configuration>
  7.               <excludes>
  8.                   <exclude>**/log4j.properties</exclude>
  9.               </excludes>
  10.               <archive>
  11.                   <manifest>
  12.                       <addClasspath>true</addClasspath>                                            
  13.                       <mainClass>com.favtuts.io.utils.FileResourcesUtils</mainClass>                      
  14.                       <classpathPrefix>dependency-jars/</classpathPrefix>
  15.                   </manifest>
  16.               </archive>
  17.           </configuration>
  18.         </plugin>

附言：此示例使用 Maven 插件创建 JAR 文件。

要检查 jar 文件中的主类，请执行以下操作：

  1. $ jar tf target/java-io.jar | more

3. 从资源文件夹获取文件 – 单元测试

3.1 我们将测试资源放在单元测试的src/test/resources文件夹中。通常，测试资源中的文件将复制到target/test-classes文件夹。

format_png 1

src/test/resources/json/file1.json

  1. {
  2.   "name": "unit test",
  3.   "age": 38
  4. }

src/test/resources/database.properties.properties

  1. datasource.url=jdbc:mysql://localhost/test?useSSL=false
  2. datasource.username=test
  3. datasource.password=password
  4. datasource.driver-class-name=com.mysql.jdbc.Driver

3.2 它的工作方式与我们从src/main/resources中读取文件的方式相同。我们使用相同的getResourceAsStream和getResource方法从 src/test/resources读取文件.

FileResourcesTest.java

  1. package com.favtuts.io;
  3. import java.io.BufferedReader;
  4. import java.io.File;
  5. import java.io.IOException;
  6. import java.io.InputStream;
  7. import java.io.InputStreamReader;
  8. import java.net.URISyntaxException;
  9. import java.net.URL;
  10. import java.nio.charset.StandardCharsets;
  11. import java.nio.file.Files;
  12. import java.util.List;
  14. import org.junit.jupiter.api.DisplayName;
  15. import org.junit.jupiter.api.Test;
  17. // Unit test class
  18. public class FileResourcesTest {
  20.     @DisplayName("Test loading a JSON file")
  21.     @Test
  22.     void loadJSONTest() {
  24.         String fileName = "json/file1.json";
  26.         ClassLoader classLoader = getClass().getClassLoader();
  28.         try (InputStream inputStream = classLoader.getResourceAsStream(fileName);
  29.             InputStreamReader streamReader = new InputStreamReader(inputStream, StandardCharsets.UTF_8);
  30.             BufferedReader reader = new BufferedReader(streamReader)
  31.         ) {
  33.             String line;
  34.             while((line = reader.readLine()) != null) {
  35.                 System.out.println(line);
  36.             }
  38.         } catch (IOException e) {
  39.             e.printStackTrace();
  40.         }
  41.     }
  44.     @DisplayName("Test loading a properties file")
  45.     @Test
  46.     void loadPropTest() throws IOException, URISyntaxException {
  48.         String fileName = "database.properties";
  50.         ClassLoader classLoader = getClass().getClassLoader();
  52.         URL resource = classLoader.getResource(fileName);
  53.         if (resource == null) {
  54.             throw new IllegalArgumentException("file not found! " + fileName);
  55.         }
  57.         //File file = new File(resource.getFile());
  58.         File file = new File(resource.toURI());
  60.         List<String> lines = Files.readAllLines(file.toPath(), StandardCharsets.UTF_8);
  61.         lines.forEach(System.out::println);
  62.     }
  64. }

输出

  1. {
  2. "name": "unit test",
  3. "age": 38
  4. }
  5. datasource.url=jdbc:mysql://localhost/test?useSSL=false
  6. datasource.username=test
  7. datasource.password=password
  8. datasource.driver-class-name=com.mysql.jdbc.Driver

4. 从资源文件夹中获取所有文件。（非 JAR 环境）

如果我们不知道确切的文件名，并且想要读取所有文件，包括资源文件夹中的子文件夹文件，我们可以使用NIO Files.walk轻松访问和读取文件。

4.1 以下示例使用Files.walk从src/main/resources/json文件夹中读取所有文件：

FileResourcesUtils.java

p

  1. package com.favtuts.io.utils;
  3. import java.io.*;
  4. import java.net.URISyntaxException;
  5. import java.net.URL;
  6. import java.nio.charset.StandardCharsets;
  7. import java.nio.file.Files;
  8. import java.nio.file.Paths;
  9. import java.util.List;
  10. import java.util.stream.Collectors;
  12. public class FileResourcesUtils {
  14.     public static void main(String[] args) throws IOException {
  16.         FileResourcesUtils app = new FileResourcesUtils();
  18.         // read all files from a resources folder
  19.         try {
  21.             // files from src/main/resources/json
  22.             List<File> result = app.getAllFilesFromResource("json");
  23.             for (File file : result) {
  24.                 System.out.println("file : " + file);
  25.                 printFile(file);
  26.             }
  28.         } catch (URISyntaxException | IOException e) {
  29.             e.printStackTrace();
  30.         }
  32.     }
  34.     private List<File> getAllFilesFromResource(String folder)
  35.         throws URISyntaxException, IOException {
  37.         ClassLoader classLoader = getClass().getClassLoader();
  39.         URL resource = classLoader.getResource(folder);
  41.         // dun walk the root path, we will walk all the classes
  42.         List<File> collect = Files.walk(Paths.get(resource.toURI()))
  43.                 .filter(Files::isRegularFile)
  44.                 .map(x -> x.toFile())
  45.                 .collect(Collectors.toList());
  47.         return collect;
  48.     }
  50.     // print a file
  51.     private static void printFile(File file) {
  53.         List<String> lines;
  54.         try {
  55.             lines = Files.readAllLines(file.toPath(), StandardCharsets.UTF_8);
  56.             lines.forEach(System.out::println);
  57.         } catch (IOException e) {
  58.             e.printStackTrace();
  59.         }
  61.     }
  63. }

输出

  1. file : /home/favtuts/projects/core-java/java-io/target/classes/json/file1.json
  2. {
  3. "name": "favtuts",
  4. "age": 38
  5. }
  6. file : /home/favtuts/projects/core-java/java-io/target/classes/json/file2.json
  7. {
  8. "name": "jack",
  9. "age": 40
  10. }
  11. file : /home/favtuts/projects/core-java/java-io/target/classes/json/sub/subfile1.json
  12. {
  13. "name": "sub",
  14. "age": 99
  15. }

4.2 但是，示例 4.1 中的标准Files.walk无法直接访问 JAR 文件中的文件，请尝试在 JAR 环境中运行示例 4.1，然后它会引发FileSystemNotFoundException。

  1. $ mvn clean package
  2. $ java -jar target/java-io.jar
  3. Exception in thread "main" java.nio.file.FileSystemNotFoundException
  4. at jdk.zipfs/jdk.nio.zipfs.ZipFileSystemProvider.getFileSystem(ZipFileSystemProvider.java:169)
  5. at jdk.zipfs/jdk.nio.zipfs.ZipFileSystemProvider.getPath(ZipFileSystemProvider.java:155)
  6. at java.base/java.nio.file.Path.of(Path.java:208)
  7. at java.base/java.nio.file.Paths.get(Paths.java:97)
  8. at com.favtuts.io.utils.FileResourcesUtils.getAllFilesFromResource(FileResourcesUtils.java:128)
  9. at com.favtuts.io.utils.FileResourcesUtils.main(FileResourcesUtils.java:35)

5. 从资源文件夹中获取所有文件。（JAR 版本）

5.1 此示例演示Files.walk如何通过FileSystems和jar:file:xxx.jar URI读取 JAR 文件内的文件夹 。

这个想法是：

  • 文件使用FileSystems在 JAR 文件中遍历文件夹，并获取所有文件名，请参见getPathsFromResourceJAR()
  • 循环所有文件名，访问并打印每个文件，如示例 2.1，请参阅getFileFromResourceAsStream()

FileResourcesUtils.java

  1. package com.favtuts.io.utils;
  3. import java.io.*;
  4. import java.net.URI;
  5. import java.net.URISyntaxException;
  6. import java.nio.charset.StandardCharsets;
  7. import java.nio.file.FileSystem;
  8. import java.nio.file.FileSystems;
  9. import java.nio.file.Files;
  10. import java.nio.file.Path;
  11. import java.util.Collections;
  12. import java.util.List;
  13. import java.util.stream.Collectors;
  15. public class FileResourcesUtils {
  17.     public static void main(String[] args) throws IOException {
  19.         FileResourcesUtils app = new FileResourcesUtils();
  21.         // Sample 3 - read all files from a resources folder (JAR version)
  22.         try {
  24.             // get paths from src/main/resources/json
  25.             List<Path> result = app.getPathsFromResourceJAR("json");
  26.             for (Path path : result) {
  27.                 System.out.println("Path : " + path);
  29.                 String filePathInJAR = path.toString();
  30.                 // Windows will returns /json/file1.json, cut the first /
  31.                 // the correct path should be json/file1.json
  32.                 if (filePathInJAR.startsWith("/")) {
  33.                     filePathInJAR = filePathInJAR.substring(1, filePathInJAR.length());
  34.                 }
  36.                 System.out.println("filePathInJAR : " + filePathInJAR);
  38.                 // read a file from resource folder
  39.                 InputStream is = app.getFileFromResourceAsStream(filePathInJAR);
  40.                 printInputStream(is);
  41.             }
  43.         } catch (URISyntaxException | IOException e) {
  44.             e.printStackTrace();
  45.         }
  47.     }
  49.     // get a file from the resources folder
  50.     // works everywhere, IDEA, unit test and JAR file.
  51.     private InputStream getFileFromResourceAsStream(String fileName) {
  53.         // The class loader that loaded the class
  54.         ClassLoader classLoader = getClass().getClassLoader();
  55.         InputStream inputStream = classLoader.getResourceAsStream(fileName);
  57.         // the stream holding the file content
  58.         if (inputStream == null) {
  59.             throw new IllegalArgumentException("file not found! " + fileName);
  60.         } else {
  61.             return inputStream;
  62.         }
  64.     }
  66.     // Get all paths from a folder that inside the JAR file
  67.     private List<Path> getPathsFromResourceJAR(String folder)
  68.         throws URISyntaxException, IOException {
  70.         List<Path> result;
  72.         // get path of the current running JAR
  73.         String jarPath = getClass().getProtectionDomain()
  74.                 .getCodeSource()
  75.                 .getLocation()
  76.                 .toURI()
  77.                 .getPath();
  78.         System.out.println("JAR Path :" + jarPath);
  80.         // file walks JAR
  81.         URI uri = URI.create("jar:file:" + jarPath);
  82.         try (FileSystem fs = FileSystems.newFileSystem(uri, Collections.emptyMap())) {
  83.             result = Files.walk(fs.getPath(folder))
  84.                     .filter(Files::isRegularFile)
  85.                     .collect(Collectors.toList());
  86.         }
  88.         return result;
  90.     }
  92.     // print input stream
  93.     private static void printInputStream(InputStream is) {
  95.         try (InputStreamReader streamReader = new InputStreamReader(is, StandardCharsets.UTF_8);
  96.              BufferedReader reader = new BufferedReader(streamReader)) {
  98.             String line;
  99.             while ((line = reader.readLine()) != null) {
  100.                 System.out.println(line);
  101.             }
  103.         } catch (IOException e) {
  104.             e.printStackTrace();
  105.         }
  107.     }
  109. }

输出

  1. $ java -jar target/java-io.jar
  2. JAR Path :/C:/Users/favtuts/projects/core-java/java-io/target/java-io.jar
  3. Path : /json/file2.json
  4. filePathInJAR : json/file2.json
  5. {
  6. "name": "jack",
  7. "age": 40
  8. }
  9. Path : /json/file1.json
  10. filePathInJAR : json/file1.json
  11. {
  12. "name": "favtuts",
  13. "age": 38
  14. }
  15. Path : /json/sub/subfile1.json
  16. filePathInJAR : json/sub/subfile1.json
  17. {
  18. "name": "sub",
  19. "age": 99
  20. }

下载源代码

$ git clone https://github.com/favtuts/java-core-tutorials-examples

$ cd java-io

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