假设场景：A与B表是一对多关系，现在多个A和多个B进行批量入库。

A的主键用某个序列JOB_SEQUENCE.nextval，B表的外键字段存A表的主键。

如果A和B在同一个事务保存里，应该怎么操作呢？

之前的做法是：

    public static int newMeasurementJobId(IntHolder jobId) {
        Connection conn = getConnection();
        if (conn == null) {
        	logger.error("newMeasurementJobId:Failed to get DBConnection");
            return 0;
        }
        Statement stmt = null;
        ResultSet rset = null;
        try {
            String sql = "select PM_JOB_SEQUENCE.nextval job_id from dual";
            stmt = conn.createStatement();
            rset = stmt.executeQuery(sql);
            if (rset.next()) {
                jobId.value = rset.getInt("job_id");
            }
            return 1;
        } catch (sqlException e) {
            return 2;
        } finally {
        	closeDataBaseConnection(conn,stmt,rset);
        }
    }

这个做法，有个问题是会大量open/close 数据库的IO，频繁操作对数据库的影响是非常大的。

若用UUID做主键是没有这个问题的，因为都是本地生成的。

那么有没有什么办法一次性获取大量的序列值呢。

有个非常绝妙的办法：

select PM_JOB_SEQUENCE.nextval job_id from (select 1 from all_objects where rownum <= 10)

其中all_objects为系统表，基本上都有几十万条数据，如果不够用，可以重复执行几次。

这样就可以一次获取10条递增的唯一序列值，且不用担心并发问题，oracle数据库已经帮你规避。

可以写个简单的测试程序，测试一下并发的场景：

假设有10个程序，同时获取同一个序列的5个值，看看是不是会有重复值呢？

Test线程类：

class Test implements Runnable {
		
		private String name;
		
		private List<String> list;
		
		public Test(String name,List<String> list){
			this.name = name;
			this.list = list;
		}

		public void run() {
			String sql = "select PM_JOB_SEQUENCE.nextval job_id from (select 1 from all_objects where rownum <= 5)";
	        Connection conn = null;
	        PreparedStatement pstmt = null;
			ResultSet rs = null;
			try {
				conn = dataSource.getConnection();
				pstmt = conn.prepareStatement(sql);
				rs = pstmt.executeQuery();
				while (rs.next()) {
					list.add(name+"_"+rs.getInt("job_id"));
				}
			} catch (sqlException e) {
				e.printStackTrace();
			} finally {
				try {
					rs.close();
					pstmt.close();
					conn.close();
				} catch (sqlException e) {
					e.printStackTrace();
				}
			}
		}		
	}

	public void test() throws Exception{
		List<String> list = Collections.synchronizedList(new ArrayList<String>());
		for (int i = 0; i < 10; i++) {
			Test test = new Test("Thread"+String.valueOf(i),list);
			Thread t = new Thread(test);
			t.start();
			t.join();
		}
		for(String val : list){
			System.out.println(val);
		}
		System.out.println("===========================");
		Collections.sort(list);
		for(String value : list){
			System.out.println(value);
		}
	}

Thread0_62443290 Thread0_62443291 Thread0_62443292 Thread0_62443293 Thread0_62443294 Thread1_62443295 Thread1_62443296 Thread1_62443297 Thread1_62443298 Thread1_62443299 Thread2_62443300 Thread2_62443301 Thread2_62443302 Thread2_62443303 Thread2_62443304 Thread3_62443305 Thread3_62443306 Thread3_62443307 Thread3_62443308 Thread3_62443309 Thread4_62443310 Thread4_62443311 Thread4_62443312 Thread4_62443313 Thread4_62443314 Thread5_62443315 Thread5_62443316 Thread5_62443317 Thread5_62443318 Thread5_62443319 Thread6_62443320 Thread6_62443321 Thread6_62443322 Thread6_62443323 Thread6_62443324 Thread7_62443325 Thread7_62443326 Thread7_62443327 Thread7_62443328 Thread7_62443329 Thread8_62443330 Thread8_62443331 Thread8_62443332 Thread8_62443333 Thread8_62443334 Thread9_62443335 Thread9_62443336 Thread9_62443337 Thread9_62443338 Thread9_62443339 =========================== Thread0_62443290 Thread0_62443291 Thread0_62443292 Thread0_62443293 Thread0_62443294 Thread1_62443295 Thread1_62443296 Thread1_62443297 Thread1_62443298 Thread1_62443299 Thread2_62443300 Thread2_62443301 Thread2_62443302 Thread2_62443303 Thread2_62443304 Thread3_62443305 Thread3_62443306 Thread3_62443307 Thread3_62443308 Thread3_62443309 Thread4_62443310 Thread4_62443311 Thread4_62443312 Thread4_62443313 Thread4_62443314 Thread5_62443315 Thread5_62443316 Thread5_62443317 Thread5_62443318 Thread5_62443319 Thread6_62443320 Thread6_62443321 Thread6_62443322 Thread6_62443323 Thread6_62443324 Thread7_62443325 Thread7_62443326 Thread7_62443327 Thread7_62443328 Thread7_62443329 Thread8_62443330 Thread8_62443331 Thread8_62443332 Thread8_62443333 Thread8_62443334 Thread9_62443335 Thread9_62443336 Thread9_62443337 Thread9_62443338 Thread9_62443339