Android Thread

Single-thread model

As JCIP introduced, many UI framework is single threaded. They use thread confinement to avoid deadlock and integrity of data.

Android also use this model.

By default, all components of the same application run in the same process and thread (called the “main” thread). If an application component starts and there already exists a process for that application (because another component from the application exists), then the component is started within that process and uses the same thread of execution.

How to use

But sometimes, we want to perform some long time task like using network to download something and read from db. In order to make UI responsive, we should perform those jobs in another thread (Notice: not in the Service, which in default still run on the main thread).

When we finish the long time data loading job, we have to push the data back to our UI thread. But we shouldn’t access it directly, which volatile the single thread model and cause data races. So we should switch back to UI thread to read from worker thread (which is called ‘thread hopping’ in jcip). In order to make to fulfill it, android provides some following method:

// read data from worker thread, notice the data in work thread still need synchronization, 
// for they are shared by multiple thread
Activity.runOnUiThread(Runnable)
View.post(Runnable)
View.postDelayed(Runnable, long)

Even better, it provide AsyncTask to make it safer and easier.
For example, when you want to check the username/password of user, you have to connect to your server to verify it. In such scenario, you can put the request and verification process into a AsyncTask and the UI thread can make some animation when there is no response.

Following is some code:

    public class UserLoginTask extends AsyncTask<Void, Void, Boolean> {

        private final String mName;
        private final String mPassword;

        UserLoginTask(String name, String password) {
            mName = name;
            mPassword = password;
        }

        // backgroud thread
        @Override
        protected Boolean doInBackground(Void... params) {
            // network access.

            for (String credential : CREDENTIALS) {
                String[] pieces = credential.split(":");
                if (pieces[0].equals(mName)) {
                    // Account exists, return true if the password matches.
                    return pieces[1].equals(mPassword);
                }
            }

            // register the new account here.
            return true;
        }

        // UI thread
        @Override
        protected void onPostExecute(final Boolean success) {
            mAuthTask = null;
            showProgress(false);

            if (success) {
                // start next
                finish();
                Intent intent = new Intent(LoginActivity.this, Drawer.class);
                startActivity(intent);
            } else {
                mPasswordView.setError(getString( R.string.error_incorrect_password ));
                mPasswordView.requestFocus();
            }
        }

        @Override
        protected void onCancelled() {
            mAuthTask = null;
            showProgress(false);
        }
    }

More

1. Android AsyncTask document

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