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# 使用重新連線邏輯來連線至 Neptune 資料庫執行個體的 Java 範例
<a name="access-graph-gremlin-java-reconnect-example"></a>

以下 Java 範例示範如何使用重新連線邏輯來連線至 Gemlin 用戶端，以從非預期的中斷連線回復。

它具有下列相依性：

```
<dependency>
    <groupId>org.apache.tinkerpop</groupId>
    <artifactId>gremlin-driver</artifactId>
    <version>${gremlin.version}</version>
</dependency>

<dependency>
    <groupId>com.amazonaws</groupId>
    <artifactId>amazon-neptune-sigv4-signer</artifactId>
    <version>${sig4.signer.version}</version>
</dependency>

<dependency>
    <groupId>com.evanlennick</groupId>
    <artifactId>retry4j</artifactId>
    <version>0.15.0</version>
</dependency>
```

以下是範例程式碼：

**重要**  
 來自 Retry4j 的 `CallExecutor` 可能不是執行緒安全的程式碼。考慮讓每個執行緒使用自己的`CallExecutor`執行個體，或使用不同的重試程式庫。

**注意**  
 下列範例已更新為包含 requestInterceptor() 的使用。這已在 TinkerPop 3.6.6 中新增。在 TinkerPop 3.6.6 版之前，程式碼範例使用 handshakeInterceptor()，該版本已棄用。

```
public static void main(String args[]) {
  boolean useIam = true;

  // Create Gremlin cluster and traversal source
  Cluster.Builder builder = Cluster.build()
         .addContactPoint(System.getenv("neptuneEndpoint"))
         .port(Integer.parseInt(System.getenv("neptunePort")))
         .enableSsl(true)
         .minConnectionPoolSize(1)
         .maxConnectionPoolSize(1)
         .serializer(Serializers.GRAPHBINARY_V1D0)
         .reconnectInterval(2000);

  if (useIam) {
      builder.requestInterceptor( r -> {
         try {
            NeptuneNettyHttpSigV4Signer sigV4Signer =
                        new NeptuneNettyHttpSigV4Signer("(your region)", new DefaultAWSCredentialsProviderChain());
            sigV4Signer.signRequest(r);
         } catch (NeptuneSigV4SignerException e) {
            throw new RuntimeException("Exception occurred while signing the request", e);
         }
         return r;
      });
   }

  Cluster cluster = builder.create();

  GraphTraversalSource g = AnonymousTraversalSource
      .traversal()
      .withRemote(DriverRemoteConnection.using(cluster));

  // Configure retries
  RetryConfig retryConfig = new RetryConfigBuilder()
      .retryOnCustomExceptionLogic(getRetryLogic())
      .withDelayBetweenTries(1000, ChronoUnit.MILLIS)
      .withMaxNumberOfTries(5)
      .withFixedBackoff()
      .build();

  @SuppressWarnings("unchecked")
  CallExecutor<Object> retryExecutor = new CallExecutorBuilder<Object>()
      .config(retryConfig)
      .build();

  // Do lots of queries
  for (int i = 0; i < 100; i++){
    String id = String.valueOf(i);

    @SuppressWarnings("unchecked")
    Callable<Object> query = () -> g.V(id)
        .fold()
        .coalesce(
            unfold(),
            addV("Person").property(T.id, id))
        .id().next();

    // Retry query
    // If there are connection failures, the Java Gremlin client will automatically
    // attempt to reconnect in the background, so all we have to do is wait and retry.
    Status<Object> status = retryExecutor.execute(query);

    System.out.println(status.getResult().toString());
  }

  cluster.close();
}

private static Function<Exception, Boolean> getRetryLogic() {

  return e -> {

    Class<? extends Exception> exceptionClass = e.getClass();

    StringWriter stringWriter = new StringWriter();
    String message = stringWriter.toString();


    if (RemoteConnectionException.class.isAssignableFrom(exceptionClass)){
      System.out.println("Retrying because RemoteConnectionException");
      return true;
    }

    // Check for connection issues
    if (message.contains("Timed out while waiting for an available host") ||
        message.contains("Timed-out") && message.contains("waiting for connection on Host") ||
        message.contains("Connection to server is no longer active") ||
        message.contains("Connection reset by peer") ||
        message.contains("SSLEngine closed already") ||
        message.contains("Pool is shutdown") ||
        message.contains("ExtendedClosedChannelException") ||
        message.contains("Broken pipe") ||
        message.contains(System.getenv("neptuneEndpoint")))
    {
      System.out.println("Retrying because connection issue");
      return true;
    };

    // Concurrent writes can sometimes trigger a ConcurrentModificationException.
    // In these circumstances you may want to backoff and retry.
    if (message.contains("ConcurrentModificationException")) {
      System.out.println("Retrying because ConcurrentModificationException");
      return true;
    }

    // If the primary fails over to a new instance, existing connections to the old primary will
    // throw a ReadOnlyViolationException. You may want to back and retry.
    if (message.contains("ReadOnlyViolationException")) {
      System.out.println("Retrying because ReadOnlyViolationException");
      return true;
    }

    System.out.println("Not a retriable error");
    return false;
  };
}
```