public String generateToken(final String id) {

    Claims claims = Jwts.claims().setSubject(id);
    long nowMillis = System.currentTimeMillis();
    long expMillis = nowMillis + tokenValidity;
    Date exp = new Date(expMillis);

    return Jwts.builder().setClaims(claims).setIssuedAt(new Date(nowMillis)).setExpiration(exp)
            .signWith(SignatureAlgorithm.HS512, jwtSecret).compact();
}

现在我想为这个方法编写JUnit，我尝试如下，但是我得到了错误

@Test
    @Order(1)
    public void test_generateToken() throws JwtTokenMalformedException, JwtTokenMissingException {
        final String subject_id = "123456789";
        final Long tokenValidity = 180000L;
        final String jwtSecret = "jwtSecret";

        when(Jwts.claims().setSubject(subject_id)).thenReturn(new DefaultClaims()); //**line no: 10

        when(Jwts.builder().setClaims(claims).setIssuedAt(new Date(nowMillis)).setExpiration(exp)
                .signWith(SignatureAlgorithm.HS512, jwtSecret).compact()).thenReturn(new String());
    }

在第10行获取错误：

org.mockito.exceptions.misusing.MissingMethodInvocationException: 
when() requires an argument which has to be 'a method call on a mock'.
For example:
    when(mock.getArticles()).thenReturn(articles);

Also, this error might show up because:
1. you stub either of: final/private/equals()/hashCode() methods.
   Those methods *cannot* be stubbed/verified.
   Mocking methods declared on non-public parent classes is not supported.
2. inside when() you don't call method on mock but on some other >     object.