Spring Expression Language (SpEL) with @Value: dollar vs. hash ($ vs. #)

Spring Expression Language (SpEL) with @Value: dollar vs. hash ($ vs. #)

Are you confused about when to use ${...} compared to #{...} in Spring's Expression Language (SpEL)? You're not alone! Spring's documentation mainly uses #{...}, but there are plenty of examples that use ${...}. To make matters even more confusing, some people may have suggested using ${...} when you first started with SpEL and it seemed to work fine. So what's the difference, and which one should you use?

Let's dive into this topic and clarify things once and for all. 🤔

What's the Difference?

Both ${...} and #{...} are used for expressing values in SpEL, but they have slightly different behaviors and use cases.

1. Dollar Sign ($)

The dollar sign $ is used when you want to access a property value from the Spring environment or system properties.

In the provided example:

@Component
public class ProxyConfiguration {

    @Value("${proxy.host}")
    private String host;
    @Value("${proxy.port}")
    private String port;

    :
}

The ${proxy.host} and ${proxy.port} expressions retrieve the values of proxy.host and proxy.port properties from a property file (e.g., application.properties, application.yml, etc.).

The dollar sign form (${...}) is widely used in traditional Spring applications, especially for retrieving externalized configuration values.

2. Hash (#)

On the other hand, the hash symbol # is used when you want to evaluate an expression or invoke a bean method. It is more powerful and flexible than the dollar sign form.

For example:

@Component
public class ExampleBean {

    @Value("#{someBean.doSomething()}")
    private String result;

    :
}

In this case, #someBean.doSomething() will invoke the doSomething() method on someBean and assign the result to the result field.

The hash symbol form (#{...}) is especially useful when you need complex expressions or want to perform operations using Spring beans.

Are They the Same?

No, they are not the same. The dollar sign form (${...}) is used for accessing property values, while the hash symbol form (#{...}) is used for evaluating expressions or invoking bean methods.

Which One Should You Use?

Now that the differences are clear, which one should you use?

• If you only need to access a simple property value, such as retrieving configuration settings, use the dollar sign form (${...}). This is the conventional and recommended way.

• If you need to evaluate an expression or invoke a method, use the hash symbol form (#{...}). It provides more flexibility and power.

It's important to note that using the hash symbol form (#{...}) allows you to access the full power of SpEL, while the dollar sign form (${...}) is more limited in its capabilities.

Conclusion

In summary, the dollar sign ($) and hash symbol (#) in SpEL have different meanings and use cases. The dollar sign ($) is used for accessing property values, while the hash symbol (#) is used for evaluating expressions or invoking bean methods.

So, next time you find yourself confused about which expression to use, remember these guidelines:

• Use ${...} to access property values.

• Use #{...} to evaluate expressions or invoke bean methods.

Keep these tips in mind, and you'll be confidently using SpEL expressions in your Spring applications! 💪

If you have any further questions or want to share your experience with SpEL, leave a comment below. Happy coding! 😄👍