How do I validate a jwt token that I got from Cognito

Spring Security 5.1 introduced support for this so it's much easier to implement. See https://docs.spring.io/spring-security/site/docs/current/reference/html/jc.html#oauth2resourceserver

Basically:

1. Add dependencies as described in https://docs.spring.io/spring-security/site/docs/current/reference/html/jc.html#dependencies
2. Add yml config as described at https://docs.spring.io/spring-security/site/docs/current/reference/html5/#oauth2resourceserver-jwt-minimalconfiguration. For cognito use following url: https://cognito-idp.<region>.amazonaws.com/<YOUR_USER_POOL_ID>
3. You would probably need to edit you security config as described at https://docs.spring.io/spring-security/site/docs/current/reference/html/jc.html#oauth2resourceserver-sansboot

Use a library like java-jwt (I guess you are using Maven)

<dependency>
    <groupId>com.auth0</groupId>
    <artifactId>java-jwt</artifactId>
    <version>3.3.0</version>
</dependency>

Then:

String token = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXUyJ9.eyJpc3MiOiJhdXRoMCJ9.AbIJTDMFc7yUa5MhvcP03nJPyCPzZtQcGEp-zWfOkEE";
try {
    Algorithm algorithm = Algorithm.HMAC256("secret");
    // or
    Algorithm algorithm = Algorithm.RSA256(publicKey, privateKey);
    JWTVerifier verifier = JWT.require(algorithm)
        .withIssuer("auth0")
        .build(); //Reusable verifier instance
    DecodedJWT jwt = verifier.verify(token);
} catch (UnsupportedEncodingException exception){
    //UTF-8 encoding not supported
} catch (JWTVerificationException exception){
    //Invalid signature/claims
}

You can manually decode a jwt-token here: https://jwt.io
More info about java-jwt here: https://github.com/auth0/java-jwt