我想使用java 8 CompletionStages連續執行2個異步方法,以便第二個在第一個失敗時不會執行。但是,當我打電話thenCompose,傳遞的功能似乎開始之前,前一個功能齊全(如:兩個功能並行錯誤地執行以下是代碼:CompletionStage.thenCompose沒有連續執行
public CompletionStage<Graph> create(Payload payload) {
CompletionStage<BlobInfo> fileFuture = createFile(payload);
CompletionStage<Entity> metadataFuture = createMetadata(payload);
return fileFuture
.thenCompose(ignore -> metadataFuture)
.thenApply(entity ->
buildFromEntity(objectMapper, entity));
}
public CompletionStage<BlobInfo> createFile(Payload payload) {
return CompletableFuture.supplyAsync(() -> {
try {
return
storage.create(
BlobInfo
.newBuilder(payload.bucket, payload.name)
.build(),
payload.data.getBytes());
} catch (StorageException e) {
LOG.error("Failed to write to storage: " + e);
throw new RequestHandlerException(StatusCode.SERVER_ERROR,
"Failed to write to storage.");
}
});
}
public CompletionStage<Entity> createMetadata(Payload payload) {
return CompletableFuture.supplyAsync(() -> createSync(payload));
}
private Entity createMetadataSync(Payload payload) {
Key key = keyFactory.newKey(payload.id);
Entity.Builder entityBuilder = GraphPayload.buildEntityFromGraph(payload, key);
Entity entity = entityBuilder.build();
LOG.error("Metadata.createSync");
try {
datastore.add(entity);
} catch (DatastoreException e) {
LOG.error("Failed to write initial metadata: " + e);
throw new RequestHandlerException(StatusCode.SERVER_ERROR,
"Failed to write initial metadata.");
}
return entity;
}
OUTPUT:
16:57:47.530 [ForkJoinPool.commonPool-worker-3] ERROR com.spotify.nfgraphstore.store.FileStore - CreateFile
16:57:47.530 [ForkJoinPool.commonPool-worker-2] ERROR com.spotify.nfgraphstore.store.MetadataStore - Metadata.createSync
16:57:47.530 [ForkJoinPool.commonPool-worker-3] ERROR com.spotify.nfgraphstore.store.FileStore - Failed to write initial graph to storage: com.google.cloud.storage.StorageException: X
記錄的輸出表明Metadata.createSync是越來越執行存儲異常得到投擲之前,這一結論也被測試(未顯示),這是爲了顯示與元數據DB零個互動脫胎於如果寫入到文件存儲數據庫失敗,該測試有時會失敗,提示競爭狀況。
所以我留下來想想thenCompose不保證串行執行。但是我在java文檔中讀到的所有內容都暗示執行應該是串行的:https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CompletionStage.html#thenCompose-java.util.function.Function-
有誰知道爲什麼執行不能保證是串行的,或者推薦其他可能更符合我的意圖的函數?
...或只是'thenApplyAsync(忽略 - > createSync(有效載荷));',因爲沒有理由來包裝操作到另一個'CompletableFuture'。 – Holger
謝謝。這個答案似乎解決了這個問題。我不明白爲什麼。我添加了更多的調試語句,並且創建元數據未來的行絕對不會導致createMetadataSync被調用。所以它必須被thenCompose立即調用。那麼爲什麼在上一階段完成之前調用完成階段呢? – jsarma
感謝您的額外解答Holger。我已經證實了這一點。 – jsarma