How to set default value to a path variable?

By default PathVariable is required but you can set it optional as :

@GetMapping(value = "/{locale}", produces = MediaType.APPLICATION_JSON_VALUE)
public ResponseEntity<String> getLocale(@PathVariable(name="locale", required= 
false) String locale) {
//set english as default value if local is null   
locale = locale == null? "english": locale;
return new ResponseEntity<>(locale, HttpStatus.OK);
}

You can use required false attribute and then can check for null or empty string value. Refer this thread

getLocale(@PathVariable(name ="locale", required= false) String locale

And then check for null or empty string.

You cannot provide default value to spring path variable as of now.

You can do the following obvious thing:

@GetMapping(value = "/{locale}", produces = MediaType.APPLICATION_JSON_VALUE)
public ResponseEntity<String> getLocale(@PathVariable("locale") String locale) {
    locale = locale == null? "english": locale;
    return new ResponseEntity<>(locale, HttpStatus.OK);
}

But more appropriate is to use Spring i18n.CookieLocaleResolver, so that you do not need that path variable anymore:

    <bean id="localeResolver" class="org.springframework.web.servlet.i18n.CookieLocaleResolver">
        <property name="defaultLocale" value="en"/>
    </bean>

We can set the required property of @PathVariable to false to make it optional.

But we will also need to listen for requests that are coming without path variable.

@GetMapping(value = { "/{locale}", "/" }, produces = MediaType.APPLICATION_JSON_VALUE)
public ResponseEntity<String> getLocale(@PathVariable("locale") String locale) {
    return new ResponseEntity<>(locale, HttpStatus.OK);
}

You just need to provide default value

@GetMapping(value = "/{locale}", produces = MediaType.APPLICATION_JSON_VALUE)
public ResponseEntity<String> getLocale(@PathVariable("locale", defaultValue="english") String locale) {
    return new ResponseEntity<>(locale, HttpStatus.OK);
}