FileFormat: Add read functions that do not throw on empty files

Especially for user-facing configuration files, empty files should be
handled leniently, and these functions allows that.

Signed-off-by: Sebastian Schuberth <[email protected]>

model/src/main/kotlin/FileFormat.kt

@@ -21,6 +21,7 @@ package org.ossreviewtoolkit.model
 
 import com.fasterxml.jackson.databind.ObjectMapper
 import com.fasterxml.jackson.module.kotlin.readValue
+import com.fasterxml.jackson.module.kotlin.treeToValue
 
 import java.io.File
 
@@ -83,6 +84,23 @@ fun File.mapper() = FileFormat.forFile(this).mapper
  */
 inline fun <reified T : Any> File.readValue(): T = mapper().readValue(this)
 
+/**
+ * Use the Jackson mapper returned from [File.mapper] to read an object of type [T] from this file, or return null if
+ * the file has no content.
+ */
+inline fun <reified T : Any> File.readValueOrNull(): T? =
+    mapper().readTree(this)?.let {
+        // Parse the file in a two-step process to avoid readValue() throwing an exception on empty files. Also see
+        // https://github.com/FasterXML/jackson-databind/issues/1406#issuecomment-252676674.
+        mapper().treeToValue(it)
+    }
+
+/**
+ * Use the Jackson mapper returned from [File.mapper] to read an object of type [T] from this file, or return the
+ * [default] value if the file has no content.
+ */
+inline fun <reified T : Any> File.readValueOrDefault(default: T): T = readValueOrNull() ?: default
+
 /**
  * Use the Jackson mapper returned from [File.mapper] to write an object of type [T] to this file. [prettyPrint]
  * indicates whether to use pretty printing or not. The function also ensures that the parent directory exists.