Java編程珠璣(201903)

1、初始化一個對象的簡便方式

Student stu = new Student(){{
	setName("張三");
}};

2、重新組合後去重

List<String> names = list.stream().map(Student::getName).distinct().collect(Collectors.toList());

3、根據某一屬性對集合去重

list = list.stream()
				.collect(Collectors.collectingAndThen(Collectors.toCollection(() -> 
								new TreeSet<>(Comparator.comparing(e -> e.getName()))), ArrayList::new));

4、集合排序

// 正序
list = list.stream().sorted((a, b) -> a.getName().compareTo(b.getName())).collect(Collectors.toList());
list = list.stream().sorted(Comparator.comparing(Student::getName)).collect(Collectors.toList());
// 倒序
list = list.stream().sorted((a, b) -> -(a.getName().compareTo(b.getName()))).collect(Collectors.toList());
list = list.stream().sorted(Comparator.comparing(Student::getName).reversed()).collect(Collectors.toList());

5、獲取Sum

BigDecimal sum1 = list.stream().map(Student::getScore).reduce((a,b) -> a.add(b)).orElse(BigDecimal.ZERO);
BigDecimal sum2 = list.stream().map(Student::getScore).reduce(BigDecimal::add).orElse(BigDecimal.ZERO);

6、過濾

boolean exists = list.stream().anyMatch(e -> e.getAmount() == null || e.getAmount().compareTo(BigDecimal.ZERO) <= 0);

7、集合轉換爲數組

Long[] ids = list.toArray(new Long[]{});
Long[] ocrFileIds = rows.stream().toArray(Long[] :: new);

8、使用CountDownLatch同時跑多個任務，一起返回

List<File> countFileList = new ArrayList<>();
if (CollectionUtils.isEmpty(countFileList)) {
	return;
}
// 定義線程安全的結果集
List<File> files = Collections.synchronizedList(new ArrayList<>());
// 定義CountDownLatch線程數，有多少個請求，我們就定義多少個
CountDownLatch runningThreadNum = new CountDownLatch(countFileList.size());
for (File file : countFileList) {
    countDownLatchTaskService.getFileBytes(runningThreadNum, file, files);
}
try {
    // 調用CountDownLatch的await方法則當前主線程會等待，直到CountDownLatch類型的runningThreadNum清0
    // 每個任務處理完成會對runningThreadNum減1
    // 如果等待?分鐘後當前主線程都等不到runningThreadNum清0，則認爲超時，直接中斷，拋出中斷異常InterruptedException
    runningThreadNum.await(wait, timeUnit);
} catch (InterruptedException e) {
    // 非正常中斷應該拋出異常或返回錯誤結果
    logger.error("CountDownLatch識別線程超時", e);
    throw new ApplicationException(ResponseCode.OVERTIME_FILE_ERROR);
}


// CountDownLatchTaskService.java 核心代碼

/**
* 獲取文件數據
* @param runningThreadNum 正在運行的線程
* @param file 識別文件
* @param resultList 識別結果列表
*/
@Async("fileBytesExecutor")
public void getFileBytes(CountDownLatch runningThreadNum, File file, List<File> resultList) {
   try{
       // 如果Bytes爲空，就自己去取
       CommonService.getFileBytes(file);
       if (file.getSize() <= 0 || ArrayUtils.isEmpty(file.getBytes()) || StringUtils.isEmpty(file.getUrl())) {
           logger.info("文件不存在或文件大小爲空或文件Url爲空, fileId:::{}, url:::{}, fileSize:::{}", file.getId(), file.getUrl(), file.getSize());
           return;
       }
       resultList.add(file);
   } catch(Exception e) {
       logger.error("獲取文件數據出錯", e);
   } finally {
       // 當前線程處理完成，runningThreadNum線程數減1，此操作必須在finally中完成，避免處理異常後造成runningThreadNum線程數無法清0
       runningThreadNum.countDown();
   }
}

9、異步任務等待當前事務提交後再執行

// 等待當前事務提交後再處理異步業務，防止數據還未提交導致數據有誤
TransactionSynchronizationManager.registerSynchronization(new TransactionSynchronizationAdapter() {
     @Override
     public void afterCommit() {
   		 // 異步任務執行
         doAsyncWork();
     }
 });