From a2cbbedc65c9200fd3c2a6a698366ac431cc153d Mon Sep 17 00:00:00 2001
From: Vincent Ambo <mail@tazj.in>
Date: Fri, 26 Jun 2020 20:25:14 +0100
Subject: chore(tazjin): Move //web/blog & //web/homepage to my userdir

Change-Id: I96a2620ffb1d9e98a1d8ce7d97f2c4f58c2dbfd3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/603
Reviewed-by: tazjin <mail@tazj.in>
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 users/tazjin/blog/posts/make-object-t-again.md | 98 ++++++++++++++++++++++++++
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+A few minutes ago I found myself debugging a strange Java issue related
+to Jackson, one of the most common Java JSON serialization libraries.
+
+The gist of the issue was that a short wrapper using some types from
+[Javaslang](http://www.javaslang.io/) was causing unexpected problems:
+
+```java
+public <T> Try<T> readValue(String json, TypeReference type) {
+  return Try.of(() -> objectMapper.readValue(json, type));
+}
+```
+
+The signature of this function was based on the original Jackson
+`readValue` type signature:
+
+```java
+public <T> T readValue(String content, TypeReference valueTypeRef)
+```
+
+While happily using my wrapper function I suddenly got an unexpected
+error telling me that `Object` is incompatible with the type I was
+asking Jackson to de-serialize, which got me to re-evaluate the above
+type signature again.
+
+Lets look for a second at some code that will *happily compile* if you
+are using Jackson\'s own `readValue`:
+
+```java
+// This shouldn't compile!
+Long l = objectMapper.readValue("\"foo\"", new TypeReference<String>(){});
+```
+
+As you can see there we ask Jackson to decode the JSON into a `String`
+as enclosed in the `TypeReference`, but assign the result to a `Long`.
+And it compiles. And it failes at runtime with
+`java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long`.
+Huh?
+
+Looking at the Jackson `readValue` implementation it becomes clear
+what\'s going on here:
+
+```java
+@SuppressWarnings({ "unchecked", "rawtypes" })
+public <T> T readValue(String content, TypeReference valueTypeRef)
+    throws IOException, JsonParseException, JsonMappingException
+{
+    return (T) _readMapAndClose(/* whatever */);
+}
+```
+
+The function is parameterised over the type `T`, however the only place
+where `T` occurs in the signature is in the parameter declaration and
+the function return type. Java will happily let you use generic
+functions and types without specifying type parameters:
+
+```java
+// Compiles fine!
+final List myList = List.of(1,2,3);
+
+// Type is now myList : List<Object>
+```
+
+Meaning that those parameters default to `Object`. Now in the code above
+Jackson also explicitly casts the return value of its inner function
+call to `T`.
+
+What ends up happening is that Java infers the expected return type from
+the context of the `readValue` and then happily uses the unchecked cast
+to fit that return type. If the type hints of the context aren\'t strong
+enough we simply get `Object` back.
+
+So what\'s the fix for this? It\'s quite simple:
+
+```java
+public <T> T readValue(String content, TypeReference<T> valueTypeRef)
+```
+
+By also making the parameter appear in the `TypeReference` we \"bind\"
+`T` to the type enclosed in the type reference. The cast can then also
+safely be removed.
+
+The cherries on top of this are:
+
+1.  `@SuppressWarnings({ "rawtypes" })` explicitly disables a
+    warning that would\'ve caught this
+
+2.  the `readValue` implementation using the less powerful `Class`
+    class to carry the type parameter does this correctly: `public <T>
+    T readValue(String content, Class<T> valueType)`
+
+The big question I have about this is *why* does Jackson do it this way?
+Obviously the warning did not just appear there by chance, so somebody
+must have thought about this?
+
+If anyone knows what the reason is, I\'d be happy to hear from you.
+
+PS: Shoutout to David & Lucia for helping me not lose my sanity over
+this.
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