Java annotation with default value

Java Annotations Default Values

You can give annotation members default values that will be used if no value is specified when the annotation is applied.

A default value is specified by adding a default clause to a member’s declaration.

type member() default value ;

Here, value must be of a type compatible with type.

Here is @MyAnno rewritten to include default values:

// An annotation type declaration that includes defaults. @Retention(RetentionPolicy.RUNTIME) @interface MyAnno < String str() default "Testing"; int val() default 9000; >

This declaration gives a default value of «Testing» to str and 9000 to val.

This means that neither value needs to be specified when @MyAnno is used.

However, either or both can be given values if you want to.

Therefore, following are the four ways that @MyAnno can be used:

@MyAnno() // both str and val default @MyAnno(str = "some string") // val defaults @MyAnno(val = 100) // str defaults @MyAnno(str = "Testing", val = 100) // no defaults 

The following program demonstrates the use of default values in an annotation.

import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.reflect.Method; // An annotation type declaration that includes defaults. @Retention(RetentionPolicy.RUNTIME) @interface MyAnno < String str() default "Testing"; int val() default 9000; > public class Main < // Annotate a method using the default values. @MyAnno()// w w w . d e m o 2 s . c om public static void myMeth() < Main ob = new Main(); // Obtain the annotation for this method // and display the values of the members. try < Class c = ob.getClass(); Method m = c.getMethod("myMeth"); MyAnno anno = m.getAnnotation(MyAnno.class); System.out.println(anno.str() + " " + anno.val()); > catch (NoSuchMethodException exc) < System.out.println("Method Not Found."); > > public static void main(String args[]) < myMeth(); >>

The output is shown here: Testing 9000

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Java default value annotation in Spring

This article focuses on the Java default value for the @Value annotation in Spring. The @Value annotation resolves and injects property values into fields, constructor, or method arguments from property files. See the @Value article to know more about the annotation.

Here, we will discuss adding the default value to the annotation.

2. Default value annotation

It’s not always necessary to assign a value when you declare a field. The compiler will set fields that are declared but not initialized to a reasonable default. Usually, this default will be zero or null based on the data type.

For example in Java, int has a default value 0, string – null, boolean – false, so on.

Similarly, It is possible to provide a default value to the @Value annotated field or argument. You had to add the default value after the colon inside the $ < >placeholder.

In this annotation @Value(“$:false“>), false is the default value for reporting.required . If the Spring cannot resolve the property, then assigns the default value false.

2.1. Default value for missing property file

You can also force the default value when the property file is missing by setting the ignore-resource-no-found to true.

For Java configurations, specify true to the setIgnoreResourceNotFound function.

package com.tedblob.value; import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.ComponentScan; import org.springframework.context.annotation.Configuration; import org.springframework.context.annotation.PropertySource; import org.springframework.context.support.PropertySourcesPlaceholderConfigurer; @Configuration @ComponentScan public class ApplicationConfiguration < @Bean public static PropertySourcesPlaceholderConfigurer propertyPlaceholderConfigurer() < PropertySourcesPlaceholderConfigurer propertySourcesPlaceHolder = new PropertySourcesPlaceholderConfigurer(); propertySourcesPlaceHolder.setIgnoreResourceNotFound(true); return propertySourcesPlaceHolder; >>

For XML based configurations, specify as below

2.2. Default value for primitive types and String

You can set the default value to primitive types and strings. You can use zero-length string as default value whereas, for primitive types, you must specify default values and cannot leave it empty.

@Value(“$” – zero-length string after the colon. So the default value is an empty string.

package com.tedblob.value; import org.springframework.beans.factory.annotation.Value; import org.springframework.stereotype.Component; @Component class Reporting < private final String emailAddress; private final boolean required; private final int count; public Reporting(@Value("$") String emailAddress, @Value("$") boolean required, @Value("$") int count) < this.emailAddress = emailAddress; this.required = required; this.count = count; >public void print() < System.out.println("Email Address : " + emailAddress); System.out.println("Reporting required : " + required); System.out.println("Reporting count : " + count); >>

2.3. Default value for array annotation

Similarly you can specify comma-separated array values as default to your array placeholder as:

public Admin(@Value("$") String[] users, @Value("#>") Map roles) < this.users = new ArrayList(Arrays.asList(users)); /* [A, B, C] */ this.roles = roles; >

If the admin.users are not present in the property, then the resolver injects the default array values A, B, C into the argument users .

2.4. Default value for Map annotation

Similar to the list, you can also set a default value for your Map as below:

public Admin(@Value("$") String[] users, @Value("#>>") Map roles) < this.users = new ArrayList(Arrays.asList(users)); /* */ this.roles = roles; >

2.5. Default value with SpEL expression

@Value annotation supports SpEL expression and you can also specify default value with SpEL expression.

You must use Elvis (?:) operator to specify the default value. The below code looks for system property and environment variables. If the resolver cannot resolve the property, then injects the specified default value DOWN or NA into the arguments.

public HealthCheck(@Value(«#») String healthcheck, @Value(«#») String javaHome)

3. Conclusion

To sum up, we have seen examples for the topic Java default value annotation in Spring. We have learned to add a default value to the primitive, string, map, array, and SpEL expressions annotated with @Value.

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Java Annotations Default Values

You can give annotation members default values that will be used if no value is specified when the annotation is applied.

A default value is specified by adding a default clause to a member’s declaration.

type member() default value ;

Here, value must be of a type compatible with type.

Here is @MyAnno rewritten to include default values:

// An annotation type declaration that includes defaults. @Retention(RetentionPolicy.RUNTIME) @interface MyAnno < String str() default "Testing"; int val() default 9000; >

This declaration gives a default value of «Testing» to str and 9000 to val.

This means that neither value needs to be specified when @MyAnno is used.

However, either or both can be given values if you want to.

Therefore, following are the four ways that @MyAnno can be used:

@MyAnno() // both str and val default @MyAnno(str = "some string") // val defaults @MyAnno(val = 100) // str defaults @MyAnno(str = "Testing", val = 100) // no defaults 

The following program demonstrates the use of default values in an annotation.

import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.reflect.Method; // An annotation type declaration that includes defaults. @Retention(RetentionPolicy.RUNTIME) @interface MyAnno < String str() default "Testing"; int val() default 9000; > public class Main < // Annotate a method using the default values. @MyAnno()// w w w . d e m o 2 s . c om public static void myMeth() < Main ob = new Main(); // Obtain the annotation for this method // and display the values of the members. try < Class c = ob.getClass(); Method m = c.getMethod("myMeth"); MyAnno anno = m.getAnnotation(MyAnno.class); System.out.println(anno.str() + " " + anno.val()); > catch (NoSuchMethodException exc) < System.out.println("Method Not Found."); > > public static void main(String args[]) < myMeth(); >>

The output is shown here: Testing 9000

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Spring @Value Default Value

Spring @Value annotation is used for expression-driven dependency injection. @Value can configure default value to be used when actual value is not available for dependency injection. @Value uses $ <. >and # <. >syntax to write expression.
1. $ <. >is a property placeholder syntax. We need to use colon (:) to specify default value with it.

@Value("$") private String userName;

2. # <. >is a SpEL syntax. It can also handle property placeholders and a lot more. We need to use Elvis Operator (?:) to specify default value with it.

@Value("#") private String mySystemVal;

We read property file using @PropertySource annotated at JavaConfig and resolve dependency injection using @Value annotation. Here we will discuss to specify default values with @Value for string, boolean, integer, list and map type fields.

Contents

• Technologies Used
• 1. String Default Value
• 2. Boolean Default Value
• 3. Integer Default Value
• 4. Array Default Value
• 5. List Default Value
• 6. Map Default Value
• 7. Default null
• Complete Example
• References
• Download Source Code

Technologies Used

Find the technologies being used in our example.
1. Java 11
2. Spring 5.1.6.RELEASE
3. Spring Boot 2.1.4.RELEASE
4. Gradle 5.0
5. Eclipse 2018-09

1. String Default Value

To resolve dependency injection using @Value we use $ <. >and # <. >syntax to write expression.
1. We need to use colon (:) to specify default value with $ <. >syntax. Suppose we have a property file myproject.properties file. In our JavaConfig, we read property file with @PropertySource annotation.

@Configuration @PropertySource("classpath:myproject.properties") public class AppConfig < @Value("$") private String userName; ------ >

When cp.user.role property is not defined in property file, the default value Shree Mahesh will be injected to userName field. To specify default value with $ <. >syntax, we need not to enclose default value with single quotes (»).
If we prefix a space to default value after : , then that will be included in default value. If we suffix a space to default value before > , it will also be included in default value. Look into the following code snippet.
a. No space as prefix and suffix.

@Value("$>") private String userRole;

When cp.user.role property is not defined in property file, the default value will be injected to userRole picking it from cp.user.role.default property.

3. To specify default value with # <. >SpEL syntax, we need to use Elvis Operator (?:) as following.

@Value("#") private String mySystemVal;

In the above code snippet, for the demo, we are trying to fetch value from system properties for an unknown key. As the value will be null, so default value will be injected to mySystemVal .

2. Boolean Default Value

@Value("$") private Boolean isUserActive;

When cp.user.active property is not defined in property file, then true will be injected to isUserActive .

3. Integer Default Value

@Value("$") private Integer userAge;

4. Array Default Value

@Value("$") private String[] userSkills;

When cp.user.skills property is not defined in property file, the default array will be injected to userSkills .
In the same way, we can inject primitive datatype integer array as default value, too.

@Value("$") private int[] userSkillIds;

5. List Default Value

@Value("#'.split(',')>") private List userSkills;

When cp.user.skills property is not defined in property file, the default list will be injected to userSkills .

6. Map Default Value

@Value("#>>") private Map teamMates;

When cp.user.teamMates property is not defined in property file, the default list will be injected to teamMates .

7. Default null

@Value("$>") private String userCountry;

When cp.user.country is not defined in property file, then userCountry will be injected with null value.

Complete Example

package com.concretepage; import java.util.List; import java.util.Map; import org.springframework.beans.factory.annotation.Value; import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.Configuration; import org.springframework.context.annotation.PropertySource; @Configuration @PropertySource("classpath:myproject.properties") public class AppConfig < //Default String @Value("$") private String userName; @Value("$>") private String userRole; @Value("$") private String userCountry; //Default Boolean @Value("$") private Boolean isUserActive; //Default Integer @Value("$") private Integer userAge; //Default using SpEL with Elvis Operator (?:) @Value("#") private String mySystemVal; //Default for List @Value("#'.split(',')>") private List userSkills; //Default for Map @Value("#>>") private Map teamMates; @Bean public User getUser() < User user = new User(); user.setUserName(userName); user.setUserRole(userRole); user.setUserCountry(userCountry); user.setUserActive(isUserActive); user.setUserAge(userAge); user.setMySystemVal(mySystemVal); user.setUserSkills(userSkills); user.setTeamMates(teamMates); return user; >>

cp.user.country= USA cp.user.role.default= Contributor

package com.concretepage; import java.util.List; import java.util.Map; public class User < private String userName; private String userRole; private String userCountry; private Boolean userActive; private Integer userAge; private String mySystemVal; private ListuserSkills; private Map teamMates; //Setters and Getters >

apply plugin: ‘java’ apply plugin: ‘eclipse’ archivesBaseName = ‘spring-demo’ repositories < mavenCentral() >dependencies

package com.concretepage; import org.springframework.context.annotation.AnnotationConfigApplicationContext; public class MySpringApp < public static void main(String[] args) < AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(); ctx.scan("com.concretepage"); ctx.refresh(); User user = ctx.getBean(User.class); System.out.println("userName: " + user.getUserName()); System.out.println("userRole: " + user.getUserRole()); System.out.println("userCountry: " + user.getUserCountry()); System.out.println("userActive: " + user.getUserActive()); System.out.println("userAge: " + user.getUserAge()); System.out.println("mySystemVal: " + user.getMySystemVal()); System.out.println("userSkills: " + user.getUserSkills()); System.out.println("teamMates: " + user.getTeamMates()); ctx.registerShutdownHook(); ctx.close(); >>

userName: Shree Mahesh userRole: Contributor userCountry: USA userActive: true userAge: 30 mySystemVal: Default System Value userSkills: [Java, Spring, Angular] teamMates:

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