Vert.x: retrieve records from database - vert.x

Question

I have method called selectAll which has to return records selected from database, but when I want to return them as String it doesn't allow it and gives me:
Local variable S defined in an enclosing scope must be final or effectively final.
I tried to add final to my String but it stills the same.
Here is my selectAll() method:
public String selectAll(String table, String order) {
String S;
S = "";
this.getCnx().getConnection(res -> {
if (res.succeeded()) {
SQLConnection connection = res.result();
connection.queryWithParams(ReqSql.SELECT_ALL, new JsonArray().add(table).add(order), (ar) -> {
if (ar.failed()) {
this.sendError(500, response);
} else {
JsonArray arr = new JsonArray();
ar.result().getRows().forEach(arr::add);
S = ar.result().getRows().toString();
response.putHeader("content-type", "application/json").end(arr.encode());
}
});
}
});
}
public AsyncSQLClient getCnx(){
JsonObject mySQLClientConfig = new JsonObject()
.put("host", "localhost")
.put("database", "test")
.put("username", "test")
.put("password", "test")
.put("port", 3306);
return MySQLClient.createShared(vertx, mySQLClientConfig);
}
And I create another class for my requests:
public class ReqSql {
public static final String SELECT_ALL = "SELECT * FROM ? ORDER BY ? ASC";
}
Regards.

Answer 1

Problem is that you need to declare local variables as final if you want to use them in a Lambda expression. But you cannot assign a value to a final variable. So this will not work:
final String S = "";
this.getCnx().getConnection(res -> {
//...
S = ar.result().getRows().toString();
//...
});
Vert.x is highly asynchronous. That means that most operations, like getConnection(), will infect return immediately but their results, in this case SQLConnection, will be available to the Handler to a later point in time.
If you try to make a asynchronous result available in the main program flow that wouldn't work. What every you want to do with S is probably wrong. So if you don't need S afterwards I would convert S to a local variable.
I suggest you read about Future in the documentation. A Future is a placeholder for results of asynchronous calls. Vert.x is full of asynchronous calls. With Future you can do something like this:
Future<SQLConnection> connection = Future.future();
this.getCnx().getConnection(res -> {
if (res.succeeded()) {
logger.info("Got a connection.");
connection.complete(res.result());
} else {
connection.fail(res.cause());
}
});
You specify a Handler on a Future for retrieving the asynchronous result in a much more readable manner than callbacks/Lambdas:
connection.setHandler(res -> {
...
})