Important TIPS when reading files on classpath

You can read a resource using multiple approaches (not all work the same).

AClass.class.getClassLoader().getResourceAsStream(file);  // preferred
ClassLoader.getSystemResourceAsStream(resourcePath)
AClass.class.getResourceAsStream()

Depending on h~~~~ow the code is written you either NEED to use a leading slash in the name of the resource loaded or don’t need to use one.
- ClassLoader.getSystemResourceAsStream() expects an absolute path within the classpath, but does not allow a leading /. If you mistakenly use a leading /, it will fail to find the resource.
- In contrast, methods like Class.getResourceAsStream(“/path/to/resource”) do allow a leading /.
- So you have to know how the method you are calling is written, hopefully the JavaDocs make it clear!
- In modular environments (like Java modules) or containerized applications (like OSGi, Tomcat, Spring Boot), the system class loader may not have the same visibility as the context class loader or a class-specific class loader.
- In such environments, ClassLoader.getSystemResourceAsStream() might not be able to find resources that are loaded by other class loaders.
If a file exists in multiple locations, the first one will be used. resolved from the top of the classpath (which typical).
Resources can be relative to the class reading the resource or relative to the top of the TREE/classpath. If a the CLASSPATH has multiple directories or JAR files then EACH one of them is searched (in turn) in the order listed in the Java -cp or -classpath command line argument.

The recommendation for reading a resource is generally to prefer using AClass.class.getClassLoader().getResourceAsStream(file) over ClassLoader.getSystemResourceAsStream(resourcePath), especially in environments where class loader visibility can vary.

Sample Code

Given a source file and a settings file at

src/main/java/com/example/proj1/pkg1/Utils.java
src/main/resources/config/config.file

You can read the config.file file using:

Utils.readResource("/config/config.file");

Because of the way the code that loads the resource is written the file could also be read using.

Utils.readResource("config/config.file");

But as stated, it is recommended to use the leading slash to avoid confusion.

import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.nio.charset.StandardCharsets;
import java.util.stream.Collectors;

public class Utils {

    /**
     * Reads a file from the classpath and returns its content as a String.
     *
     * @param file The path to the file on the classpath.
     * @return The content of the file as a String.
     * @throws IllegalArgumentException If the file is not found on the classpath.
     */
    public static String readResource(String file) {
        // Try to load the file from the classpath
        InputStream inputStream = Utils.class.getClassLoader().getResourceAsStream(file);
        
        // If the file is not found, throw an exception
        if (inputStream == null) {
            throw new IllegalArgumentException("File not found on the classpath: " + file);
        }

        // Read the file content using BufferedReader
        try (BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream, StandardCharsets.UTF_8))) {
            return reader.lines().collect(Collectors.joining("\n"));
        } catch (Exception e) {
            throw new RuntimeException("Error reading file: " + file, e);
        }
    }
}

Notes: Relative vs Absolute Classpath

When reading a resource the resource can be relative to the class that is reading the resource.

For example, a class

src/main/java/com/example/proj/pkg1/MyCode.java
src/main/java/com/example/proj/pkg1/mycode1.file
src/main/resources/config/config.file
src/main/resources/com/example/proj/pkg1/mycode2.file

And the resulting compiled code that gets put into a JAR file or under target when compiling with maven or a Java IDE.

target/classes/com/example/proj/pkg1/MyCode.java
target/classes/com/example/proj/pkg1/mycode2.file
target/classes/config/config.file

NOTE: mycode1.file will not (by default) be copied to the target/classes directory if you intended to have that file read as a resource then you should move it to the src/main/resources directory or make update the pom.xml file to include it (using instructions below)

To update the pom.xml file to copy

<build>
    <resources>
        <resource>
            <directory>src/main/java</directory>
            <includes>
                <include>**/*.file</include>
            </includes>
        </resource>
    </resources>
</build>

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