Add fromJson JNI method for Policy object

CedarJava/src/main/java/com/cedarpolicy/model/policy/Policy.java

@@ -85,6 +85,11 @@ public String toJson() throws InternalException, NullPointerException {
         return toJsonJni(policySrc);
     }
 
+    public static Policy fromJson(String policyId, String policyJson) throws InternalException, NullPointerException {
+        var policyText = fromJsonJni(policyJson);
+        return new Policy(policyText, policyId);
+    }
+
     public static Policy parseStaticPolicy(String policyStr) throws InternalException, NullPointerException {
         var policyText = parsePolicyJni(policyStr);
         return new Policy(policyText, null);
@@ -100,4 +105,5 @@ private static native String parsePolicyTemplateJni(String policyTemplateStr)
             throws InternalException, NullPointerException;
 
     private native String toJsonJni(String policyStr) throws InternalException, NullPointerException;
+    private static native String fromJsonJni(String policyJsonStr) throws InternalException, NullPointerException;
 }

CedarJava/src/test/java/com/cedarpolicy/PolicyTests.java

@@ -91,4 +91,22 @@ public void policyTemplateToJsonFailureTests() throws InternalException {
             assertTrue(e.getMessage().contains("expected a static policy, got a template containing the slot ?resource"));
         }
     }
+
+    @Test
+    public void policyFromJsonTest() throws InternalException {
+        assertThrows(NullPointerException.class, () -> {
+            String nullJson = null;
+            Policy.fromJson(null, nullJson);
+        });
+        assertThrows(InternalException.class, () -> {
+            String invalidJson = "effect\":\"permit\",\"principal\":{\"op\":\"All\"},\"action\":{\"op\":\"All\"}";
+            Policy.fromJson(null, invalidJson);
+        });
+
+        String validJson = "{\"effect\":\"permit\",\"principal\":{\"op\":\"All\"},\"action\":{\"op\":\"All\"},"
+                + "\"resource\":{\"op\":\"All\"},\"conditions\":[]}";
+        Policy p = Policy.fromJson(null, validJson);
+        String actualJson = p.toJson();
+        assertEquals(validJson, actualJson);
+    }
 }

CedarJavaFFI/src/interface.rs

@@ -386,6 +386,34 @@ fn to_json_internal<'a>(env: &mut JNIEnv<'a>, policy_jstr: JString<'a>) -> Resul
     }
 }
 
+#[jni_fn("com.cedarpolicy.model.policy.Policy")]
+pub fn fromJsonJni<'a>(mut env: JNIEnv<'a>, _: JClass, policy_json_jstr: JString<'a>) -> jvalue {
+    match from_json_internal(&mut env, policy_json_jstr) {
+        Err(e) => jni_failed(&mut env, e.as_ref()),
+        Ok(policy_text) => policy_text.as_jni(),
+    }
+}
+
+fn from_json_internal<'a>(
+    env: &mut JNIEnv<'a>,
+    policy_json_jstr: JString<'a>,
+) -> Result<JValueOwned<'a>> {
+    if policy_json_jstr.is_null() {
+        raise_npe(env)
+    } else {
+        let policy_json_jstring = env.get_string(&policy_json_jstr)?;
+        let policy_json_string = String::from(policy_json_jstring);
+        let policy_json_value: Value = serde_json::from_str(&policy_json_string)?;
+        match Policy::from_json(None, policy_json_value) {
+            Err(e) => Err(Box::new(e)),
+            Ok(p) => {
+                let policy_text = format!("{}", p);
+                Ok(JValueGen::Object(env.new_string(&policy_text)?.into()))
+            }
+        }
+    }
+}
+
 #[jni_fn("com.cedarpolicy.value.EntityIdentifier")]
 pub fn getEntityIdentifierRepr<'a>(mut env: JNIEnv<'a>, _: JClass, obj: JObject<'a>) -> jvalue {
     match get_entity_identifier_repr_internal(&mut env, obj) {