Using global variables in cucumber scenarios spring

I'm writing my tests with cucumber for a spring boot api. And within different scenarios of the same feature, i want to use a global variable by initializing it at first, give it value in the first scenario, then use that variable new value later in the next scenarios. Problem is, whenever a scenario ends, the global variable retrieves its first initialized value while i need the new one. Any suggestions?

public class MyStepDef {

   String testString = "FIRST VALUE";

   @Given("^first given$")
   public void firstGiven()  {
       // ....
   }

   @Then("^first then$")
   public void firstThen() {
     // changing 'testString' value
       testString = "NEW VALUE";
       // ....
   }

   @Given("^second given$")
   public void secondGiven()  {
       // ....
   }

   @Then("^second then$")
   public void secondThen() {
       // using 'testString' variable : expected : 'NEW VALUE', FOUND : 'OLD VALUE'
       // ....
   }
}

Feature: MyFeature

  Scenario: first scenario
    Given first given
    Then first then

  Scenario: second scenario
    Given second given
    Then second then

        <dependency>
            <groupId>io.cucumber</groupId>
            <artifactId>cucumber-java</artifactId>
            <version>2.3.1</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>io.cucumber</groupId>
            <artifactId>cucumber-junit</artifactId>
            <version>2.3.1</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>io.cucumber</groupId>
            <artifactId>cucumber-spring</artifactId>
            <version>2.3.1</version>
            <scope>test</scope>
        </dependency>