Mateusz Pachulski

Android from the foundation · Start here

A running app is a process

When you open an app, the phone starts it as a separate program with its own private data. That's why one app crashing doesn't take the others down.

The problem

You've built an app. But what actually happens when someone taps its icon? And why can a crashing game not take your notes app down with it? Everything in this course starts here.

See it

Two running apps are two processes, each with its own memory; one crashes and the other keeps running Notes Music memorymemory "Buy milk" ♪ song Musiccrashed still running

  1. On the phone, an app you've installed is just files. Tap the Notes icon, and the phone starts it running: Notes is now a process.

    Step 1 of 4

  2. Open Music too, and you have two processes, side by side.

    Step 2 of 4

  3. Each process keeps its data while it runs, like the note you're typing, in its own memory. Predict: what happens to your note if Music crashes?

    Step 3 of 4

  4. Nothing. Music's process ends, and Notes keeps running with its memory untouched. Processes are kept apart.

    Step 4 of 4

Save this diagram as a GIF

The building block

A process is a running program. When you tap an app's icon, the phone starts it as a process; when the phone closes it, the process ends. The app's files stay installed either way.

Each process has its own memory: the data the program keeps while it runs, like the text you're typing or the list on the screen. One process can't see or change another process's own data in memory.

The analogy

Each running app is a house. Its memory is everything inside the house. Neighbours live in their own houses: they can't walk into yours, and if a house burns down, the street is still standing.

Check yourself

The music app crashes while your notes app is open. What happens to the note you were typing?

Right. A crash ends only that process. Other processes keep their own memory.

Not quite. The music app can't reach the notes app's memory at all: different houses.

Not quite. Each process has its own memory. A crash ends one process; the others don't even notice.

Is an app you installed but haven't opened a process?

Not quite. Installing puts the app's files on the phone. A process exists only while the app runs.

Right. A process is a running program. Installed but not running means files only.

Why can't one app read the text another app is showing?

Right. Each process's memory is private to that process.

Not quite. The language doesn't matter: the walls are around each process's memory.

Sources

What you now know

An installed app is just files. A running app is a process, with its own memory that no other process can touch.