How to read a properties file in Java? (Code Example)
Java needs to use the Properties class to cooperate with InputStream to read the properties file. It supports two methods: class path (getResourceAsStream) and file system path (Files.newInputStream). It needs to handle exceptions, close the stream, and can set default values and type conversions.
To read a properties file in Java, use java.util.Properties with an InputStream . The standard approach loads key-value pairs from a plain-text .properties file (eg, config.properties ) into a Properties object, then retrieve values by key.
Loading a properties file from the classpath
This is the most common case — when the file is in src/main/resources (Maven/Gradle) or on the classpath.
- Use
getClass().getResourceAsStream("/config.properties")— leading/means root of classpath - Always close the stream (preferably with try-with-resources)
- Handle
IOExceptionand missing file gracefully
Properties props = new Properties();
try (InputStream input = MyClass.class.getResourceAsStream("/config.properties")) {
if (input == null) {
throw new IllegalArgumentException("config.properties not found in classpath");
}
props.load(input);
} catch (IOException e) {
throw new RuntimeException("Failed to load config.properties", e);
}
String dbUrl = props.getProperty("database.url");
String username = props.getProperty("database.username");Loading a properties file from a file system path
Use this when the file is outside the application (eg, /etc/myapp/config.properties or a relative path).
- Use
Files.newInputStream(Paths.get("config.properties")) - Check if the file exists and is readable before loading
- Prefer absolute paths for production; resolve relative paths carefully
Path configPath = Paths.get("config.properties");
Properties props = new Properties();
try (InputStream input = Files.newInputStream(configPath)) {
props.load(input);
} catch (IOException e) {
throw new RuntimeException("Cannot read " configPath, e);
}Reading properties with default values and type safety
getProperty() returns String , but apps often need numbers, booleans, or fallbacks.
- Use
getProperty(key, defaultValue)to avoidnull - Parse manually:
Boolean.parseBoolean(props.getProperty("debug", "false")) - For production apps, consider a small utility method or library like Apache Commons Configuration
Notes on properties file format
A valid .properties file supports:
- Plain key=value lines (
app.name=MyApp) - Comments with
#or!at start of line - Backslash escaping:
message=Hello\ World→"Hello World" - Unicode escapes like
\u00E9(for non-ASCII characters) - No built-in support for nested structure or lists — those require custom parsing
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