how to use utc in postgres timestamp with jdbc Pre

I'am using timestamp data type on pg9.4, but there come very strange problem with to_json.

now i am in Shanghai, UTC+08:00 timezone.

see below:

    conn.createStatement().execute("set time zone 'UTC'");

    String sql = "select to_json(?::timestamp) as a, to_json(current_timestamp::timestamp) as b";
    PreparedStatement ps = conn.prepareStatement(sql);
    Timestamp timestamp = new Timestamp(new Date().getTime());      
    ps.setTimestamp(1, timestamp);

    ResultSet rs = ps.executeQuery();
    while(rs.next()){
        System.out.println("a " + rs.getString("a") + ", b " + rs.getString("b"));
    }

output: a "2015-09-24T16:52:42.529", b "2015-09-24T08:53:25.468191"

it's mean when i pass a TIMESTAMP parameter to pg with jdbc, the timezone is still in shanghai, not UTC.

this problem is not due to to_json function, i have make a table with one timestamp column, this problem still exits, the code of above is shortest sample.

how to let's all timestamp work in UTC timezone?

You need to set Calendar tzCal = Calendar.getInstance(TimeZone.getTimeZone("UTC"));, Before you create your prepared statement.

UPDATED CODE SNIPPET

    conn.createStatement().execute("set time zone 'UTC'");

    String sql = "select to_json(?::timestamp) as a, to_json(current_timestamp::timestamp) as b";

    Calendar tzCal = Calendar.getInstance(TimeZone.getTimeZone("UTC"));

    PreparedStatement ps = conn.prepareStatement(sql);
    Timestamp timestamp = new Timestamp(new Date().getTime());      
    ps.setTimestamp(1, timestamp);

    ResultSet rs = ps.executeQuery();
    while(rs.next()){
        System.out.println("a " + rs.getString("a") + ", b " + rs.getString("b"));
    }

This way you will be able to set timezone to UTC in your JDBC call.

If you want to run the whole application/JVM in UTC, set -Duser.timezone=UTC flag while starting JVM.

HTH.