Getting error in future get method in multithreading-CodePudding

I am trying multithreading in java for the first time. My problem statement is to fetch the data from an api for multiple months together. Therefor I tried 2-3 things and stuck with the below code. I am getting null value while printing f.get(). Anyone who can explain why it is printing null value for all?

class StyleThreadDemo implements Callable<String>{
    
    private Integer n;
    public StyleThreadDemo(int a){
        this.setN(a);
    }
    public String callApi() throws Exception {
        String output=null;
        try {
        URL url = new URL("https://api.publicapis.org/entries");
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("GET");
        conn.setRequestProperty("Accept", "application/json");

        if (conn.getResponseCode() != 200) {
            throw new RuntimeException("Failed : HTTP error code : "
                      conn.getResponseCode());
        }

        BufferedReader br = new BufferedReader(new InputStreamReader(
            (conn.getInputStream())));

        
        System.out.println("Data starting coming .... \n");
        while ((output = br.readLine()) != null) {
            System.out.println("result "  n );
        }

        conn.disconnect();
        }catch(Exception e) {
            e.printStackTrace();
        }
        return output;
    }

    @Override
    public String call() throws Exception {
        String s=callApi();
        
        return s;
    }
    public Integer getN() {
        return n;
    }
    public void setN(Integer n) {
        this.n = n;
    }
}

public class StyleThread {
    public static void shutdownAndAwaitTermination(ExecutorService executorService) {
        executorService.shutdown();
        try {
            if (!executorService.awaitTermination(60, TimeUnit.SECONDS)) {
                executorService.shutdownNow();
            } 
        } catch (InterruptedException ie) {
            executorService.shutdownNow();
            Thread.currentThread().interrupt();
        }
    }
     public static void main(String[] args) throws Exception
        {
         StyleThread styleThread = new StyleThread();
         
            ExecutorService pool = Executors.newFixedThreadPool(30);  
            
            int n = 5; // Number of threads
            List<StyleThreadDemo> tasks= new ArrayList<StyleThreadDemo>();
//          List<Future<String>> futures = new ArrayList<>();

            for (int i = 0; i < n; i  ) {
//              Future<String> result= pool.submit(new StyleThreadDemo(i));
//              futures.add(result);
                
               tasks.add(new StyleThreadDemo(i));
               System.out.println("task added " i);
               
            }
          
            List<Future<String>> futures=pool.invokeAll(tasks); 
            shutdownAndAwaitTermination(pool);
            
            for (Future<String> f : futures) {
                  
                  System.out.println("printing : f.get());
                }
           
            
            
            
        }

}

CodePudding user response：

your probleme is here :

while ((output = br.readLine()) != null) {
      System.out.println("result "  n );
}
[...]
return output;

when you are getting out your loop output is null

solution

StringBuilder sb = new StringBuilder();
while ((output = br.readLine()) != null) {
      System.out.println("result "  n );
      sb.append(output);
}
[...]
return sb.toString();

CodePudding user response：

br.readLine() returns null when the end of the stream is reached. So you eventually assign null to output.

try:

output = new String(conn.getInputStream().readAllBytes(), StandardCharsets.UTF_8);