{"id":289,"date":"2021-12-29T04:45:39","date_gmt":"2021-12-29T04:45:39","guid":{"rendered":"https:\/\/datmt.com\/?p=289"},"modified":"2022-03-25T04:58:32","modified_gmt":"2022-03-25T04:58:32","slug":"java-concurrency-03-callable","status":"publish","type":"post","link":"https:\/\/datmt.com\/backend\/java-concurrency-03-callable\/","title":{"rendered":"[Java Concurrency] 03: Callable"},"content":{"rendered":"\n

In the previous article, you’ve already known about the Runnable<\/a><\/code> interface. To execute a task, you can simply create a class implement Runnable<\/code> and pass an instance of that class in a thread.<\/p>\n\n\n\n

Runnable<\/code> instances don’t provide a return value, you may need a different solution if your use case requires capturing return values.<\/p>\n\n\n\n

Introducing Callable<\/h2>\n\n\n\n

Similar to Runnable, the Callable<\/a> interface is a functional interface. One of the key differences is you can return a value if your class implement Callable.<\/p>\n\n\n\n

Let’s create a class implementing Callable:<\/p>\n\n\n\n

package com.datmt.concurrency.java.simpletasks;\n\nimport java.util.concurrent.Callable;\n\npublic class D_SimpleCallableTask implements Callable<String> {\n    private static int instanceCount;\n    @Override\n    public String call() throws Exception {\n        try {\n\n            System.out.println("START: D_SimpleCallableTask in "  + Thread.currentThread().getName() + " with instance #" + instanceCount);\n            Thread.sleep(1000);\n\n            System.out.println("DONE: D_SimpleCallableTask in "  + Thread.currentThread().getName()  + " with instance #" + instanceCount);\n\n        } catch (InterruptedException ex) {\n            ex.printStackTrace();\n            return null;\n        } finally {\n            instanceCount++;\n        }\n        return  "Done calling in D_SimpleCallableTask at instance count: " + instanceCount;\n\n    }\n}\n<\/pre><\/div>\n\n\n\n
As you can see, the class D_SimpleCallableTask<\/code> emulates a long running process (takes 1 second) and then returns a string.<\/p>\n\n\n\n
But how do we get the return result of a callable?<\/p>\n\n\n\n
Getting the result of callables<\/h2>\n\n\n\n
To execute a Callable instance, we don’t use thread. Instead, we will use an ExecutorService instance. Don’t worry if you don’t know about ExecutoreService yet. We will cover that in a later article.<\/p>\n\n\n\n
package com.datmt.concurrency.java.executor;\n\nimport com.datmt.concurrency.java.simpletasks.D_SimpleCallableTask;\n\nimport java.util.concurrent.*;\n\npublic class D_Executor {\n\n\n    public static void main(String[] args) throws InterruptedException, ExecutionException {\n\n        ExecutorService service = Executors.newSingleThreadExecutor();\n        Callable<String> task1 = new D_SimpleCallableTask();\n\n        Future<String> task1Result = service.submit(task1);\n\n        System.out.println("Task 1 result: " + task1Result.get());\n\n    }\n}\n<\/pre><\/div>\n\n\n\n
As you can see, the submit<\/code> method of ExecutorService<\/code> takes a Callable<\/code> instance and returns a Future<\/code>.<\/p>\n\n\n\n
To get the result, simply call the get<\/code> method on the Future<\/code> instance. This method will wait, if necessary, to get the result of the call method of the Callable<\/code> instance.<\/p>\n\n\n\n
Throwing exceptions in Callable<\/h2>\n\n\n\n
Unlike  Runnable, you can throw exceptions in Callable instances and catch them later in the caller’s thread.<\/p>\n\n\n\n
For example, this code will NOT<\/strong> compile:<\/p>\n\n\n\n
package com.datmt.concurrency.java.simpletasks;\n\npublic class D_SimpleRunnableTask implements Runnable {\n\n    private static int instanceCount = 0;\n    @Override\n    public void run() {\n        try {\n\n            System.out.println("START: D_SimpleVoidTask in "  + Thread.currentThread().getName() + " with instance #" + instanceCount);\n            Thread.sleep(1000);\n\n            System.out.println("DONE: D_SimpleVoidTask in "  + Thread.currentThread().getName()  + " with instance #" + instanceCount);\n            \/\/ Throwing exception is NOT OK in Runnable\n            if (instanceCount < 1)\n                throw  new Exception("Because I want to");\n            instanceCount++;\n        } catch (InterruptedException ex) {\n            ex.printStackTrace();\n        }\n    }\n}\n<\/pre><\/div>\n\n\n\n
But this will:<\/p>\n\n\n\n
package com.datmt.concurrency.java.simpletasks;\n\nimport java.util.concurrent.Callable;\n\npublic class D_SimpleCallableTask implements Callable<String> {\n    private static int instanceCount;\n    @Override\n    public String call() throws Exception {\n        try {\n\n            System.out.println("START: D_SimpleCallableTask in "  + Thread.currentThread().getName() + " with instance #" + instanceCount);\n            Thread.sleep(1000);\n\n            System.out.println("DONE: D_SimpleCallableTask in "  + Thread.currentThread().getName()  + " with instance #" + instanceCount);\n            \/\/ Throwing exception is OK in Callable\n            if (instanceCount < 1)\n                throw  new Exception();\n        } catch (InterruptedException ex) {\n            ex.printStackTrace();\n            return null;\n        } finally {\n            instanceCount++;\n        }\n        return  "Done calling in D_SimpleCallableTask at instance count: " + instanceCount;\n\n    }\n}\n<\/pre><\/div>\n\n\n\n
In the caller’s thread, you can catch the exception like this:<\/p>\n\n\n\n
package com.datmt.concurrency.java.executor;\n\nimport com.datmt.concurrency.java.simpletasks.D_SimpleCallableTask;\n\nimport java.util.concurrent.*;\n\npublic class D_Executor {\n\n\n    public static void main(String[] args) throws InterruptedException, ExecutionException {\n\n        ExecutorService service = Executors.newSingleThreadExecutor();\n        Callable<String> task1 = new D_SimpleCallableTask();\n        Callable<String> task2 = new D_SimpleCallableTask();\n\n        try {\n            Future<String> task1Result = service.submit(task1);\n\n            System.out.println("Task 1 result: " + task1Result.get());\n        } catch (Exception e) {\n            System.out.println("got an exception " + e.getMessage());\n        }\n\n        Future<String> task2Result = service.submit(task2);\n\n        System.out.println("Task 2 result: " + task2Result.get());\n    }\n}\n<\/pre><\/div>\n\n\n\n
Even when task1<\/code> threw an exception, task2<\/code> still running and produces the expected result:<\/p>\n\n\n\n
\"Throwing<\/figure>\n\n\n\n
<\/p>\n\n\n\n
Conclusion<\/h2>\n\n\n\n
Callable is a very powerful interface that allows to execute long-running tasks and get the result. You can also throw exceptions in Callable and catch it later in the caller’s thread.<\/p>\n","protected":false},"excerpt":{"rendered":"
In the previous article, you’ve already known about the Runnable interface. To execute a task, you can simply create a class implement Runnable and pass an instance of that class in a thread. Runnable instances don’t provide a return value, you may need a different solution if your use case requires capturing return values. Introducing … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"inline_featured_image":false,"footnotes":""},"categories":[1],"tags":[],"class_list":["post-289","post","type-post","status-publish","format-standard","hentry","category-backend"],"yoast_head":"\n[Java Concurrency] 03: Callable - datmt<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/datmt.com\/backend\/java-concurrency-03-callable\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"[Java Concurrency] 03: Callable - datmt\" \/>\n<meta property=\"og:description\" content=\"In the previous article, you’ve already known about the Runnable interface. 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