Introduction

Combining multiple property values in a Spring Boot application can sometimes lead to confusion, especially when leveraging properties files with SpEL (Spring Expression Language). In this article, we will address common issues encountered while trying to construct a combined value from existing property variables and provide a working example.

Overview of the Issue

You are attempting to set a property value.c based on two other properties value.a and value.b. The intention is to output a single string of the format a:b, where a and b are dynamically assigned based on either environment variables or default values in your properties file. However, your current implementation results in a parsing error. Let’s dive deeper into the root cause and provide a functional solution.

Understanding the Error

From the error log you're seeing:

InvocationTargetException: Error creating bean with name 'MyProperties': Unsatisfied dependency expressed through method 'setValueC' parameter 0; nested exception is org.springframework.beans.factory.BeanExpressionException: Expression parsing failed; nested exception is org.springframework.expression.spel.SpelParseException...

This indicates a parsing issue with your SpEL expression. Specifically, Spring seems to be having trouble interpreting the syntax you used for combining the values. The use of hashes (#) is incorrect in the context provided.

Step-by-Step Solution

To resolve this, we must change how we assemble the value.c. Let’s explore a cleaner way to accomplish this.

Update the Properties File

Your properties file should remain relatively unchanged, but the way value.c is defined needs enhancement:

my.value.a=a
my.value.b=b
value.a = #{systemEnvironment['MY_VALUE_A'] != null ? systemEnvironment['MY_VALUE_A'] : '${my.value.a}'}
value.b = #{systemEnvironment['MY_VALUE_B'] != null ? systemEnvironment['MY_VALUE_B'] : '${my.value.b}'}
value.c = #{systemEnvironment['MY_VALUE_C'] != null ? systemEnvironment['MY_VALUE_C'] : '$\{value.a\}:$\{value.b\}'}

Notice we use a simple string concatenation and escape the ${} syntax properly. This tells Spring to evaluate each property properly while concatenating them with a colon.

Modify the Java Class

Here’s how your Java class can look, ensuring we are still using the @Value annotation effectively:

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Component;

@Component
public class MyProperties {
    private String a;
    private String b;
    private String c;

    @Value("${value.a}")
    public void setValueA(String value) {
        this.a = value;
    }

    @Value("${value.b}")
    public void setValueB(String value) {
        this.b = value;
    }

    @Value("${value.c}")
    public void setValueC(String value) {
        this.c = value;
    }

    // Getters for accessing values
    public String getA() { return a; }
    public String getB() { return b; }
    public String getC() { return c; }
}

Explanation of Changes

Instead of trying to combine value.a and value.b within a Spring expression directly, the updated properties file syntax properly uses the placeholders which can be resolved by Spring at runtime. Now Spring will safely concatenate these two values when assigning to value.c.

Conclusion

By correcting the syntax in your properties file, you can successfully combine multiple property values in a Spring Boot application without encountering parsing errors. The final output of value.c will yield the intended result of a:b as expected, provided the corresponding environment variables are set correctly. Be sure to test the application thoroughly to validate that all configurations work as expected.

Frequently Asked Questions

Can I use multiple properties in a single SpEL expression?

Yes, you can combine multiple properties in SpEL, but ensure you are using the correct syntax to avoid parsing issues.

What should I do if I encounter further SpEL parsing errors?

Double-check your syntax against Spring’s SpEL documentation to ensure there are no misplaced characters or incorrect expressions. Adapt the use of ${} vs. #{} based on your needs.

How do I access the combined property in my application?

Once defined in the class using @Value, you can access it through getter methods or directly in the desired service/component where it is injected, ensuring to utilize the Spring context.