• Java – 从资源文件夹中读取文件

    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。

    getResourceAsStream方法返回一个InputStream .​​​​​​​

    1.   // the stream holding the file content
    2.   InputStream is = getClass().getClassLoader().getResourceAsStream("file.txt");
    3.  
    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 {
    6.  
    7.       // failed if files have whitespaces or special characters
    8.       //return new File(resource.getFile());
    9.  
    10.       return new File(resource.toURI());
    11.   }
    12.  
    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。

    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。​​​​​​​​​​​​​​

    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;
    2.  
    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;
    13.  
    14. public class FileResourcesUtils {
    15.  
    16.     public static void main(String[] args) throws Exception {
    17.  
    18.         FileResourcesUtils app = new FileResourcesUtils();
    19.  
    20.         //String fileName = "database.properties";
    21.         String fileName = "json/file1.json";
    22.  
    23.         System.out.println("getResourceAsStream : " + fileName);
    24.         InputStream is = app.getFileFromResourceAsStream(fileName);
    25.         printInputStream(is);
    26.  
    27.         System.out.println("\ngetResource : " + fileName);
    28.         File file = app.getFileFromResource(fileName);
    29.         printFile(file);
    30.  
    31.     }
    32.  
    33.     // get a file from the resources folder
    34.     // works everywhere, IDEA, unit test and JAR file.
    35.     private InputStream getFileFromResourceAsStream(String fileName) {
    36.  
    37.         // The class loader that loaded the class
    38.         ClassLoader classLoader = getClass().getClassLoader();
    39.         InputStream inputStream = classLoader.getResourceAsStream(fileName);
    40.  
    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.         }
    47.  
    48.     }
    49.  
    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{
    58.  
    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 {
    64.  
    65.             // failed if files have whitespaces or special characters
    66.             //return new File(resource.getFile());
    67.  
    68.             return new File(resource.toURI());
    69.         }
    70.  
    71.     }
    72.  
    73.     // print input stream
    74.     private static void printInputStream(InputStream is) {
    75.  
    76.         try (InputStreamReader streamReader =
    77.                     new InputStreamReader(is, StandardCharsets.UTF_8);
    78.              BufferedReader reader = new BufferedReader(streamReader)) {
    79.  
    80.             String line;
    81.             while ((line = reader.readLine()) != null) {
    82.                 System.out.println(line);
    83.             }
    84.  
    85.         } catch (IOException e) {
    86.             e.printStackTrace();
    87.         }
    88.  
    89.     }
    90.  
    91.     // print a file
    92.     private static void printFile(File file) {
    93.  
    94.         List 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.         }
    101.  
    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.(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

    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.         
    2.         <plugin>
    3.           <groupId>org.apache.maven.pluginsgroupId>
    4.           <artifactId>maven-jar-pluginartifactId>
    5.           <version>3.2.0version>
    6.           <configuration>
    7.               <excludes>
    8.                   <exclude>**/log4j.propertiesexclude>
    9.               excludes>
    10.               <archive>
    11.                   <manifest>
    12.                       <addClasspath>trueaddClasspath>                                            
    13.                       <mainClass>com.favtuts.io.utils.FileResourcesUtilsmainClass>                      
    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：

    $ 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。

    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;
    2.  
    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;
    13.  
    14. import org.junit.jupiter.api.DisplayName;
    15. import org.junit.jupiter.api.Test;
    16.  
    17. // Unit test class
    18. public class FileResourcesTest {
    19.  
    20.     @DisplayName("Test loading a JSON file")
    21.     @Test
    22.     void loadJSONTest() {
    23.  
    24.         String fileName = "json/file1.json";
    25.  
    26.         ClassLoader classLoader = getClass().getClassLoader();
    27.  
    28.         try (InputStream inputStream = classLoader.getResourceAsStream(fileName);
    29.             InputStreamReader streamReader = new InputStreamReader(inputStream, StandardCharsets.UTF_8);
    30.             BufferedReader reader = new BufferedReader(streamReader)
    31.         ) {
    32.             
    33.             String line;
    34.             while((line = reader.readLine()) != null) {
    35.                 System.out.println(line);
    36.             }
    37.  
    38.         } catch (IOException e) {
    39.             e.printStackTrace();
    40.         }
    41.     }
    42.  
    43.  
    44.     @DisplayName("Test loading a properties file")
    45.     @Test
    46.     void loadPropTest() throws IOException, URISyntaxException {
    47.  
    48.         String fileName = "database.properties";
    49.  
    50.         ClassLoader classLoader = getClass().getClassLoader();
    51.  
    52.         URL resource = classLoader.getResource(fileName);
    53.         if (resource == null) {
    54.             throw new IllegalArgumentException("file not found! " + fileName);
    55.         }
    56.  
    57.         //File file = new File(resource.getFile());
    58.         File file = new File(resource.toURI());
    59.  
    60.         List lines = Files.readAllLines(file.toPath(), StandardCharsets.UTF_8);
    61.         lines.forEach(System.out::println);
    62.     }
    63.     
    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;
    2.  
    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;
    11.  
    12. public class FileResourcesUtils {
    13.  
    14.     public static void main(String[] args) throws IOException {
    15.  
    16.         FileResourcesUtils app = new FileResourcesUtils();
    17.  
    18.         // read all files from a resources folder
    19.         try {
    20.  
    21.             // files from src/main/resources/json
    22.             List result = app.getAllFilesFromResource("json");
    23.             for (File file : result) {
    24.                 System.out.println("file : " + file);
    25.                 printFile(file);
    26.             }
    27.  
    28.         } catch (URISyntaxException | IOException e) {
    29.             e.printStackTrace();
    30.         }
    31.  
    32.     }
    33.  
    34.     private List getAllFilesFromResource(String folder)
    35.         throws URISyntaxException, IOException {
    36.  
    37.         ClassLoader classLoader = getClass().getClassLoader();
    38.  
    39.         URL resource = classLoader.getResource(folder);
    40.  
    41.         // dun walk the root path, we will walk all the classes
    42.         List collect = Files.walk(Paths.get(resource.toURI()))
    43.                 .filter(Files::isRegularFile)
    44.                 .map(x -> x.toFile())
    45.                 .collect(Collectors.toList());
    46.  
    47.         return collect;
    48.     }
    49.  
    50.     // print a file
    51.     private static void printFile(File file) {
    52.  
    53.         List 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.         }
    60.  
    61.     }
    62.  
    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;
    2.  
    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;
    14.  
    15. public class FileResourcesUtils {
    16.  
    17.     public static void main(String[] args) throws IOException {
    18.  
    19.         FileResourcesUtils app = new FileResourcesUtils();
    20.  
    21.         // Sample 3 - read all files from a resources folder (JAR version)
    22.         try {
    23.  
    24.             // get paths from src/main/resources/json
    25.             List result = app.getPathsFromResourceJAR("json");
    26.             for (Path path : result) {
    27.                 System.out.println("Path : " + path);
    28.  
    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.                 }
    35.  
    36.                 System.out.println("filePathInJAR : " + filePathInJAR);
    37.  
    38.                 // read a file from resource folder
    39.                 InputStream is = app.getFileFromResourceAsStream(filePathInJAR);
    40.                 printInputStream(is);
    41.             }
    42.  
    43.         } catch (URISyntaxException | IOException e) {
    44.             e.printStackTrace();
    45.         }
    46.  
    47.     }
    48.  
    49.     // get a file from the resources folder
    50.     // works everywhere, IDEA, unit test and JAR file.
    51.     private InputStream getFileFromResourceAsStream(String fileName) {
    52.  
    53.         // The class loader that loaded the class
    54.         ClassLoader classLoader = getClass().getClassLoader();
    55.         InputStream inputStream = classLoader.getResourceAsStream(fileName);
    56.  
    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.         }
    63.  
    64.     }
    65.  
    66.     // Get all paths from a folder that inside the JAR file
    67.     private List getPathsFromResourceJAR(String folder)
    68.         throws URISyntaxException, IOException {
    69.  
    70.         List result;
    71.  
    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);
    79.  
    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.         }
    87.  
    88.         return result;
    89.  
    90.     }
    91.  
    92.     // print input stream
    93.     private static void printInputStream(InputStream is) {
    94.  
    95.         try (InputStreamReader streamReader = new InputStreamReader(is, StandardCharsets.UTF_8);
    96.              BufferedReader reader = new BufferedReader(streamReader)) {
    97.  
    98.             String line;
    99.             while ((line = reader.readLine()) != null) {
    100.                 System.out.println(line);
    101.             }
    102.  
    103.         } catch (IOException e) {
    104.             e.printStackTrace();
    105.         }
    106.  
    107.     }
    108.  
    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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  • 原文地址：https://blog.csdn.net/allway2/article/details/127128371