can we store json object/any object in postgres database without any pojo/entity class in springboot?

There are several ways within the SpringBoot ecosystem to do what you are describing, but the most basic would be using JdbcTemplate Starting with a table that looks like this

create table foo (bar jsonb);

with one inserted json object:

insert into foo values ('{ "firstName": "Aurthur", "lastName": "Dent" }');

An method with a Spring-injected datasource would look something like this:

@Component
public class JsonbExample {
  @Autowired DataSource dataSource;

public void fetchJsonField() {
    var jdbcTemplate = new JdbcTemplate(dataSource);

    List<JsonNode> nodeList = jdbcTemplate.query("select bar from foo", (RowMapper<JsonNode>) (rs, rowNum) -> {
      try {
        var obj = rs.getObject("bar", PGobject.class);
        return new ObjectMapper().readValue(obj.getValue(), JsonNode.class);
      } catch (JsonProcessingException e) {
        log.error(e);
        return null;
      }
    });
    System.out.println(nodeList);
  }  

The output will be

[{"lastName":"Dent","firstName":"Aurthur"}]

Notice that the column defined as postgres type jsonb gets returned as type PGObject