Data from JdbcIO.read() cannot pass to next transform on ApexRunner

public static void testJDBCRead(Pipeline pipeline) {
    System.out.println("in testJDBCRead()");
    pipeline.apply(JdbcIO.<String>read()
            .withDataSourceConfiguration(JdbcIO.DataSourceConfiguration.create(
                    "com.mysql.jdbc.Driver", "jdbc:mysql://localhost:3307/libra")
                    .withUsername("root")
                    .withPassword("123456"))
            .withQuery("SELECT * FROM o_flow_account_login limit 3")
            .withCoder(StringUtf8Coder.of())
            .withRowMapper(new JdbcIO.RowMapper<String>() {
                public String mapRow(ResultSet resultSet) throws Exception {
                    System.out.println("maprow");
                    return "tmp";
                }
            })

    )

            .apply(ParDo.of(new DoFn<String, String>() {
                        @ProcessElement
                        public void process(ProcessContext context) {
                            System.out.println("??");
                            context.output(" ");
                        }
                    }));
}

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