Oppiskele Reading Properties Files in Java | Introduction to Java Properties

The Properties class in Java provides a simple way to manage configuration data stored in key-value pairs within .properties files. To load a properties file, you typically use the load(InputStream inStream) method, which reads the file content from an input stream and populates the Properties object with the keys and values. If the file is missing or the stream cannot be opened, an IOException is thrown, so it is important to handle such exceptions appropriately.

Once loaded, you can retrieve property values using getProperty(String key), which returns the value as a String or null if the key is not found. To provide a fallback, you can use getProperty(String key, String defaultValue), which returns the specified default value if the key is missing. This approach helps prevent NullPointerException when accessing configuration keys that may not always be present.

If you try to load a file that does not exist, or if there is a syntax error in the file, the load method will throw an exception. Therefore, always handle file access and parsing errors gracefully, ideally by logging meaningful error messages or providing sensible defaults.

Main.java

config.properties


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package com.example;

import java.io.FileInputStream;
import java.io.IOException;
import java.util.Properties;

public class Main {
    public static void main(String[] args) {
        Properties props = new Properties();
        try (FileInputStream fis = new FileInputStream("config.properties")) {
            props.load(fis);
            String host = props.getProperty("db.host", "localhost");
            String port = props.getProperty("db.port", "3306");
            String user = props.getProperty("db.user", "root");
            String password = props.getProperty("db.password", ""); // no default for security

            System.out.println("Database Host: " + host);
            System.out.println("Database Port: " + port);
            System.out.println("Database User: " + user);
            System.out.println("Database Password: " + (password.isEmpty() ? "(not set)" : password));
        } catch (IOException e) {
            System.err.println("Could not load properties file: " + e.getMessage());
        }
    }
}

When reading properties files, you may encounter missing files, keys that are absent, or invalid file formats. To handle these situations, always wrap file loading in a try-catch block to manage IOException. If a required key is missing, use the overloaded getProperty method with a default value to ensure your application continues to function. For critical configuration, you might want to check if the returned value is null and handle it explicitly, perhaps by throwing a custom exception or logging an error. It is also good practice to validate the loaded values, especially if they are required for your application to start correctly.

Main.java

app.properties


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package com.example;

import java.io.FileInputStream;
import java.io.IOException;
import java.util.Properties;
import java.util.Set;

public class Main {
    public static void main(String[] args) {
        Properties props = new Properties();
        try (FileInputStream fis = new FileInputStream("app.properties")) {
            props.load(fis);
            Set<String> keys = props.stringPropertyNames();
            for (String key : keys) {
                String value = props.getProperty(key);
                System.out.println(key + " = " + value);
            }
        } catch (IOException e) {
            System.err.println("Failed to read properties file: " + e.getMessage());
        }
    }
}

1. What happens if you try to access a property key that does not exist in the loaded properties file?

2. Which method is used to load a properties file from an InputStream in Java?

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Main.java

config.properties


              1234567891011121314151617181920212223242526
            
package com.example;

import java.io.FileInputStream;
import java.io.IOException;
import java.util.Properties;

public class Main {
    public static void main(String[] args) {
        Properties props = new Properties();
        try (FileInputStream fis = new FileInputStream("config.properties")) {
            props.load(fis);
            String host = props.getProperty("db.host", "localhost");
            String port = props.getProperty("db.port", "3306");
            String user = props.getProperty("db.user", "root");
            String password = props.getProperty("db.password", ""); // no default for security

            System.out.println("Database Host: " + host);
            System.out.println("Database Port: " + port);
            System.out.println("Database User: " + user);
            System.out.println("Database Password: " + (password.isEmpty() ? "(not set)" : password));
        } catch (IOException e) {
            System.err.println("Could not load properties file: " + e.getMessage());
        }
    }
}

Main.java

app.properties


              1234567891011121314151617181920212223
            
package com.example;

import java.io.FileInputStream;
import java.io.IOException;
import java.util.Properties;
import java.util.Set;

public class Main {
    public static void main(String[] args) {
        Properties props = new Properties();
        try (FileInputStream fis = new FileInputStream("app.properties")) {
            props.load(fis);
            Set<String> keys = props.stringPropertyNames();
            for (String key : keys) {
                String value = props.getProperty(key);
                System.out.println(key + " = " + value);
            }
        } catch (IOException e) {
            System.err.println("Failed to read properties file: " + e.getMessage());
        }
    }
}

1. What happens if you try to access a property key that does not exist in the loaded properties file?

2. Which method is used to load a properties file from an InputStream in Java?

Oliko kaikki selvää?

Kiitos palautteestasi!

Osio 1. Luku 2