JUnit JSON Snapshot Testing

Minimal JUnit extension for JSON snapshot tests

Inspired by Java Snapshot Testing from Origin Energy and the aim to not add more dependencies.

It supports

• automatic creation of missing snapshots
• automatic updates of wrong snapshots
• multiple snapshots per test method
• pretty-printed JSON
• ignoring specific fields in JSON comparison

Nothing more. No flexible configuration or customization.

Example

1. Write a snapshot test like the following.

   package com.example;
   
   // imports
   
   @ExtendWith(JsonSnapshotExtension.class)
   public class SnapshotTest {
   
     private ExpectJsonSnapshot expectJsonSnapshot;
   
     @Test
     void testMatchesSnapshots() {
       LocalDateTime indeterministicValue = LocalDateTime.now();
       String json1 =
         "{\"createdAt\": \"" + indeterministicValue + "\", \"field\": \"value\"}";
       String json2 = "{\"otherField\": 1}";
       
       expectJsonSnapshot.withIgnoredFields("createdAt").toMatch(json1);
       
       expectJsonSnapshot.toMatch(json2);
     }
   }

2. To automatically create or update snapshots on test run, add a json-snapshot.properties file in the test resources dir with content

   update-snapshots=true
   

3. Run the test to then find the snapshots. For the example test this should be a snapshot file testMatchesSnapshots.json in dir test/resources/snapshots/com/example/JsonSnapshotExtensionTest

   {
     "createdAt": "2024-10-19T16:35:46.708845800",
     "field": "value"
   }

   and a second snapshot file testMatchesSnapshots2.json in the same dir.

   {
     "otherField": 1
   }

4. To disable automatic creation or update of snapshots, change the json-snapshot.properties file to

   update-snapshots=false
   

5. Run the test. It will now assert that the JSON produced in the test matches the JSON from the snapshot file. 🎉

Note

Ignored fields are included in the snapshot, just ignored for comparison.
So if you update snapshots then the ignored fields will also be updated in the snapshots.