what is sql injection and how to prevent it code example

Example: how to prevent sql injection in java

public List<AccountDTO>  unsafeFindAccountsByCustomerId(String customerId)  throws SQLException {    // UNSAFE !!! DON'T DO THIS !!!    String sql = "select "      + "customer_id,acc_number,branch_id,balance "      + "from Accounts where customer_id = '"      + customerId       + "'";    Connection c = dataSource.getConnection();    ResultSet rs = c.createStatement().executeQuery(sql);    // ...}