多线程（无状态类）(Multithreading (Stateless Classes))

 为长代码发帖道歉，但我想知道是否有人可以帮助解决多线程问题（我对多线程很陌生）。 我正在尝试设计一个可以与多个线程共享的RESTFUL Web服务API的Facade类。 我正在使用HttpURLConnection进行连接，而Google GSON则用于转换JSON数据和从JSON数据转换。 
 
 以下是我到目前为止的课程。 在这个例子中，它有一个公共方法来进行API调用（authenticateCustomer（）），私有方法用于促进API调用（即构建POST数据字符串，发出POST请求等）。 
 
 我创建了这个类的一个实例并与1000个线程共享它。 线程调用authenticateCustomer（）方法。 大多数线程都可以工作，但有些线程会获得空指针异常，这是因为我没有实现任何同步。 如果我使authenticateCustomer（）方法'同步'它的工作原理。 问题是这导致并发性差（例如，POST请求突然需要很长时间才能完成，这将阻止所有其他线程）。 
 
 现在问我的问题。 下面的类不是无状态的，因此是线程安全的吗？ 类中的极少数字段被声明为final，并在构造函数中赋值。 所有方法都使用局部变量。 Gson对象是无状态的（根据他们的网站）并且无论如何都在API方法中创建为局部变量。 
 
public final class QuizSyncAPIFacade 
{
    // API Connection Details
private final String m_apiDomain;
private final String m_apiContentType;
private final int m_bufferSize;

// Constructors
public QuizSyncAPIFacade()
{
    m_apiDomain      = "http://*****************************";
    m_apiContentType = ".json";
    m_bufferSize = 8192; // 8k
}

private String readInputStream(InputStream stream) throws IOException
{
        // Create a buffer for the input stream
    byte[] buffer = new byte[m_bufferSize];

    int readCount;

    StringBuilder builder = new StringBuilder();

    while ((readCount = stream.read(buffer)) > -1) {
        builder.append(new String(buffer, 0, readCount));
    }

    return builder.toString();
}

private String buildPostData(HashMap<String,String> postData) throws UnsupportedEncodingException
{
    String data = "";

    for (Map.Entry<String,String> entry : postData.entrySet()) 
    {
        data += (URLEncoder.encode(entry.getKey(), "UTF-8") + "=" + URLEncoder.encode(entry.getValue(), "UTF-8") + "&");        
    }

    // Trim the last character (a trailing ampersand)
    int length = data.length();

    if (length > 0) {
        data = data.substring(0, (length - 1));
    }

    return data;
}

private String buildJSONError(String message, String name, String at)
{
    String error = "{\"errors\":[{\"message\":\"" + message + "\",\"name\":\"" + name + "\",\"at\":\"" + at + "\"}]}";

    return error;
}

private String callPost(String url, HashMap<String,String> postData) throws IOException
{
    // Set up the URL for the API call 
    URL apiUrl = new URL(url);

    // Build the post data
    String data = buildPostData(postData);

    // Call the API action
    HttpURLConnection conn;

    try {
        conn = (HttpURLConnection)apiUrl.openConnection();
    } catch (IOException e) {
        throw new IOException(buildJSONError("Failed to open a connection.", "CONNECTION_FAILURE", ""));
    }

    // Set connection parameters for posting data
    conn.setRequestMethod("POST");
    conn.setUseCaches(false);
    conn.setDoInput(true);
    conn.setDoOutput(true);

    // Write post data
    try {
        DataOutputStream wr = new DataOutputStream(conn.getOutputStream());
        wr.writeBytes(data);
        wr.flush();
        wr.close();
    } catch (IOException e) {
        throw new IOException(buildJSONError("Failed to post data in output stream (Connection OK?).", "POST_DATA_FAILURE", ""));           
    }

    // Read the response from the server                
    InputStream is;

    try {
        is = conn.getInputStream();
    } catch (IOException e) {
        InputStream errStr = conn.getErrorStream();

        if (errStr != null) 
        {
            String errResponse = readInputStream(errStr);
            throw new IOException(errResponse);
        } 
        else 
        {
            throw new IOException(buildJSONError("Failed to read error stream (Connection OK?).", "ERROR_STREAM_FAILURE", ""));
        }
    }

    // Read and return response from the server
    return readInputStream(is);
}

/* -------------------------------------
 * 
 * Synchronous API calls
 * 
   ------------------------------------- */

public APIResponse<CustomerAuthentication> authenticateCustomer(HashMap<String,String> postData)
{
    // Set the URL for this API call
    String apiURL = m_apiDomain + "/customer/authenticate" + m_apiContentType;

    Gson jsonConv = new Gson();

    String apiResponse = "";

    try 
    { 
        // Call the API action
        apiResponse = callPost(apiURL, postData);

        // Convert JSON response to the required object type
        CustomerAuthentication customerAuth = jsonConv.fromJson(apiResponse, CustomerAuthentication.class);

        // Build and return the API response object
        APIResponse<CustomerAuthentication> result = new APIResponse<CustomerAuthentication>(true, customerAuth, null);

        return result;
    } 
    catch (IOException e) 
    {
        // Build and return the API response object for a failure with error list
        APIErrorList errorList = jsonConv.fromJson(e.getMessage(), APIErrorList.class);

        APIResponse<CustomerAuthentication> result = new APIResponse<CustomerAuthentication>(false, null, errorList);

        return result;
    }
}
 
 } 

Apologies for the long code post but am wondering if someone can help with a multithreading question (I am quite new to multi-threading). I am trying to design a facade class to a RESTFUL web services API that can be shared with multiple threads. I am using HttpURLConnection to do the connection and Google GSON to convert to and from JSON data. 
 
The below class is what I have so far. In this example it has one public method to make an API call (authenticateCustomer()) and the private methods are used to facilitate the API call (i.e to build the POST data string, make a POST request etc).
 
I make one instance of this class and share it with 1000 threads. The threads call the authenticateCustomer() method. Most of the threads work but there is some threads that get a null pointer exception which is because I haven't implemented any synchronization. If I make the authenticateCustomer() method 'synchronized' it works. The problem is this results in poor concurrency (say, for example, the POST request suddenly takes a long time to complete, this will then hold up all the other threads).
 
Now to my question. Is the below class not stateless and therefore thread-safe? The very few fields that are in the class are declared final and assigned in the constructor. All of the methods use local variables. The Gson object is stateless (according to their web site) and created as a local variable in the API method anyway. 
 
public final class QuizSyncAPIFacade 
{
    // API Connection Details
private final String m_apiDomain;
private final String m_apiContentType;
private final int m_bufferSize;

// Constructors
public QuizSyncAPIFacade()
{
    m_apiDomain      = "http://*****************************";
    m_apiContentType = ".json";
    m_bufferSize = 8192; // 8k
}

private String readInputStream(InputStream stream) throws IOException
{
        // Create a buffer for the input stream
    byte[] buffer = new byte[m_bufferSize];

    int readCount;

    StringBuilder builder = new StringBuilder();

    while ((readCount = stream.read(buffer)) > -1) {
        builder.append(new String(buffer, 0, readCount));
    }

    return builder.toString();
}

private String buildPostData(HashMap<String,String> postData) throws UnsupportedEncodingException
{
    String data = "";

    for (Map.Entry<String,String> entry : postData.entrySet()) 
    {
        data += (URLEncoder.encode(entry.getKey(), "UTF-8") + "=" + URLEncoder.encode(entry.getValue(), "UTF-8") + "&");        
    }

    // Trim the last character (a trailing ampersand)
    int length = data.length();

    if (length > 0) {
        data = data.substring(0, (length - 1));
    }

    return data;
}

private String buildJSONError(String message, String name, String at)
{
    String error = "{\"errors\":[{\"message\":\"" + message + "\",\"name\":\"" + name + "\",\"at\":\"" + at + "\"}]}";

    return error;
}

private String callPost(String url, HashMap<String,String> postData) throws IOException
{
    // Set up the URL for the API call 
    URL apiUrl = new URL(url);

    // Build the post data
    String data = buildPostData(postData);

    // Call the API action
    HttpURLConnection conn;

    try {
        conn = (HttpURLConnection)apiUrl.openConnection();
    } catch (IOException e) {
        throw new IOException(buildJSONError("Failed to open a connection.", "CONNECTION_FAILURE", ""));
    }

    // Set connection parameters for posting data
    conn.setRequestMethod("POST");
    conn.setUseCaches(false);
    conn.setDoInput(true);
    conn.setDoOutput(true);

    // Write post data
    try {
        DataOutputStream wr = new DataOutputStream(conn.getOutputStream());
        wr.writeBytes(data);
        wr.flush();
        wr.close();
    } catch (IOException e) {
        throw new IOException(buildJSONError("Failed to post data in output stream (Connection OK?).", "POST_DATA_FAILURE", ""));           
    }

    // Read the response from the server                
    InputStream is;

    try {
        is = conn.getInputStream();
    } catch (IOException e) {
        InputStream errStr = conn.getErrorStream();

        if (errStr != null) 
        {
            String errResponse = readInputStream(errStr);
            throw new IOException(errResponse);
        } 
        else 
        {
            throw new IOException(buildJSONError("Failed to read error stream (Connection OK?).", "ERROR_STREAM_FAILURE", ""));
        }
    }

    // Read and return response from the server
    return readInputStream(is);
}

/* -------------------------------------
 * 
 * Synchronous API calls
 * 
   ------------------------------------- */

public APIResponse<CustomerAuthentication> authenticateCustomer(HashMap<String,String> postData)
{
    // Set the URL for this API call
    String apiURL = m_apiDomain + "/customer/authenticate" + m_apiContentType;

    Gson jsonConv = new Gson();

    String apiResponse = "";

    try 
    { 
        // Call the API action
        apiResponse = callPost(apiURL, postData);

        // Convert JSON response to the required object type
        CustomerAuthentication customerAuth = jsonConv.fromJson(apiResponse, CustomerAuthentication.class);

        // Build and return the API response object
        APIResponse<CustomerAuthentication> result = new APIResponse<CustomerAuthentication>(true, customerAuth, null);

        return result;
    } 
    catch (IOException e) 
    {
        // Build and return the API response object for a failure with error list
        APIErrorList errorList = jsonConv.fromJson(e.getMessage(), APIErrorList.class);

        APIResponse<CustomerAuthentication> result = new APIResponse<CustomerAuthentication>(false, null, errorList);

        return result;
    }
}
 
}

原文：https://stackoverflow.com/questions/10447969