What is Jetifier?

Assuming that you are familiar with AndroidX. If not, please see @this post.

Jetifier will convert support libraries of all your dependencies to AndroidX automatically, if you don't set it to true then your project will have both, the support (got deprecated after 28.0.0 version) and AndroidX package, which is redundant.

For Example

If you have PhotoView.java in your dependency. That uses support library AppCompatImageView.

import android.support.v7.widget.AppCompatImageView;

This class is moved now to androidx package, so how will PhotoView get androidx AppCompatImageView? And app still runs in device.

Who made this run ?

Jetifier, which converts all support package of dependency at build time.

Jetifier will convert android.support.v7.widget.AppCompatImageView to androidx.appcompat.widget.AppCompatImageView while building the project.

Conclusion

Enabling Jetifier is important when you migrate from Support Libraries to AndroidX.

See this post to understand more about AndroidX.

Info

Your code may show compile time errors after enabling Jetifier while using dependency classes. which you can remove by deleting .idea, .gradle and re-sync project.

This year's Google I/O (18), Google has announced Jetpack which is set/collection of libraries to make developer's life easier.

Jetpack includes previously introduced Android architecture components (ViewModel, Room, Paging, LiveData etc.) as well as newly introduced architecture components like WorkManager, Navigation. Apart from this Jetpack also has other set of libraries like AndroidX, AndroidKTX etc.

AndroidX is new package structure for Android support libraries like support, databinding, design etc.

e.g. now on wards developers will use androidx.databinding. instead of android.databinding. while importing libraries in our projects

This enables Google to add SemVer or Semantic Versioning in there library packages. For developers, this means we don't have to use same support library version for all support libraries. Every support or better to say AndroidX library will maintain its own versioning.

Another advantages for developers is that we don't have to care about maintaining same version for all support library in our project.

About Jetifier, it converts all support package of dependency at build time. As per official documentation of Jetifier

Jetifier tool migrates support-library-dependent libraries to rely on the equivalent AndroidX packages instead. The tool lets you migrate an individual library directly, instead of using the Android gradle plugin bundled with Android Studio.

To use AndroidX in a project we have to set targetSdkVersion for our project to 28 and add following 2 lines in gradle.properties file.

android.useAndroidX=true

android.enableJetifier=true

I hope this will answer your query.

EDIT

This link has mapping of all support library component with their AndroidX counter part.

Also please refer This blog for detailed explanation about AndroidX

Jetifier helps in migrating an android project to AndroidX.

Jetifier helps in making 3rd party library compatible with your AndroidX project.

If you're not using any 3rd party library in your android project, then you don't need to use Jetifier because Jetifier helps only in "making 3rd party library" compatible with your project in case you're migrating to AndroidX. So to make your project use Jetifier, you write below code in gradle.properties.

android.enableJetifier=true
android.useAndroidX=true

If you're not migrating to AndroidX and using the previous way of using Android libraries(e.g: com.android.support), then these 3rd party libraries need not be made compatible as these 3rd party libraries are already compatible with the previous way of using Android libraries. In that case, you don't need to use Jetifier, So now, you don't need to write lines given above or you can simply write as below to change the values in future to move to androidx:-

android.enableJetifier=false
android.useAndroidX=false

Jetifier

From official doc

When this flag is set to true, the Android plugin automatically migrates existing third-party libraries to use AndroidX dependencies by rewriting their binaries. The flag is false by default if it is not specified.

It is useful for compatibility. When a Consumer uses androidX and a Producer uses support. For example when your project on AndroidX wants to use a dependency which uses support library instead of AndroidX^[Example]

Jetifier

The jetifier is a handy tool that automatically migrates your dependencies to AndroidX at build time. Without it, you'd need every dependency you use to have an AndroidX version before you can migrate, and that probably won't happen for some time.

There's an important limitation to note: the jetifier only works on packaged artifacts. It does not work on your source code, which you are expected to update yourself

for enabling AndroidX, you need to add 2 flags to your gradle.properties file. The first flag tells the Android Plugin to use AndroidX packages instead of AppCompat, and the second flag will enable the Jetifier :

android.useAndroidX=true
android.enableJetifier=true

See this, this post to understand more about AndroidX