1. Overview
Spring’s @Value annotation provides a convenient way to inject property values into components. It’s also quite useful to provide sensible defaults for cases where a property may not be present.
That’s what we’re going to be focusing on in this tutorial — how to specify a default value for the @Value Spring annotation.
For a more detailed quick guide on @Value, see the article here.
Learn to use the Spring @Value annotation to configure fields from property files, system properties, etc.
Tutorial for how to work with properties files and property values in Spring.
This article explores Spring Expression Language (SpEL), a powerful expression language that supports querying and manipulating object graphs at runtime.
2. String Defaults
Let’s look at the basic syntax for setting a default value for a String property:
@Value("${some.key:my default value}")
private String stringWithDefaultValue;
If some.key cannot be resolved, stringWithDefaultValue will be set to the default value of my default value.
Similarly, we can set a zero-length String as the default value:
@Value("${some.key:}")
private String stringWithBlankDefaultValue;
3. Primitives
To set a default value for primitive types such as boolean and int, we use the literal value:
@Value("${some.key:true}")
private boolean booleanWithDefaultValue;
@Value("${some.key:42}")
private int intWithDefaultValue;
If we wanted to, we could use primitive wrappers instead by changing the types to Boolean and Integer.
4. Arrays
We can also inject a comma separated list of values into an array:
@Value("${some.key:one,two,three}")
private String[] stringArrayWithDefaults;
@Value("${some.key:1,2,3}")
private int[] intArrayWithDefaults;
In the first example above, the values one, two and three are injected as defaults into stringArrayWithDefaults.
In the second example, the values 1, 2Â and 3 are injected as defaults into intArrayWithDefaults.
5. Using SpEL
We can also use Spring Expression Language (SpEL) to specify an expression and a default.
In the example below, we expect some.system.key to be set as a system property, and if it is not set, we want to use my default system property value as a default:
@Value("#{systemProperties['some.key'] ?: 'my default system property value'}")
private String spelWithDefaultValue;
6. Conclusion
In this quick article, we looked at how we can set a default value for a property whose value we would like to have injected using Spring’s @Value annotation.
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