如何并行而不是顺序执行多个查询？

如何并行而不是顺序执行多个查询？

如果您能够使用Java
8，则可以

parallelStream

对表列表进行 *** 作，并使用lambda将表名扩展为每个表的唯一ID对应列表，然后将结果合并为一个表哈希。

没有Java 8，我将使用Google Guava的可监听期货和类似以下内容的执行服务：

public static Set<String> fetchFromTable(int table) {    String sql = "select * from testkeyspace.test_table_" + table + ";";    Set<String> result = new HashSet<String>();    // populate result with your SQL statements    // ...    return result;}public static Set<String> fetchFromAllTables() throws InterruptedException, ExecutionException {    // Create a ListeningExecutorService (Guava) by wrapping a     // normal ExecutorService (Java)     ListeningExecutorService executor =  MoreExecutors.listeningDecorator(Executors.newCachedThreadPool());    List<ListenableFuture<Set<String>>> list =  new ArrayList<ListenableFuture<Set<String>>>();     // For each table, create an independent thread that will     // query just that table and return a set of user IDs from it    for (int i = 0; i < 10; i++) {        final int table = i;        ListenableFuture<Set<String>> future = executor.submit(new Callable<Set<String>>() { public Set<String> call() throws Exception {     return fetchFromTable(table); }        });        // Add the future to the list        list.add(future);    }    // We want to know when ALL the threads have completed,     // so we use a Guava function to turn a list of ListenableFutures    // into a single ListenableFuture    ListenableFuture<List<Set<String>>> combinedFutures = Futures.allAsList(list);    // The get on the combined ListenableFuture will now block until     // ALL the individual threads have completed work.    List<Set<String>> tableSets = combinedFutures.get();    // Now all we have to do is combine the individual sets into a    // single result    Set<String> userList = new HashSet<String>();    for (Set<String> tableSet: tableSets) {        userList.addAll(tableSet);    }    return userList;}

Executors和Futures的使用都是Java的核心。番石榴唯一要做的就是让我将Future变成ListenableFutures。请参阅此处以讨论为何后者更好。

可能仍有改善这种方法并行性的方法，但是如果您花费大量时间等待数据库响应或处理网络流量，则此方法可能会有所帮助。