Gson taking a lock on SimpleDateFormat

[ferozed]

While debugging a problem in our service, I noticed that Gson is using SimpleDateFormat. While doing that, it is taking a lock that causes other threads using that same instance of Gson to block...

2020-06-22_19:00:26.64218    java.lang.Thread.State: BLOCKED (on object monitor)
2020-06-22_19:00:26.64218       at com.google.gson.DefaultDateTypeAdapter.serialize(DefaultDateTypeAdapter.java:71)
2020-06-22_19:00:26.64218       - waiting to lock <0x00000000d016dc68> (a java.text.SimpleDateFormat)
2020-06-22_19:00:26.64219       at com.google.gson.DefaultDateTypeAdapter.serialize(DefaultDateTypeAdapter.java:37)
2020-06-22_19:00:26.64219       at com.google.gson.internal.bind.TreeTypeAdapter.write(TreeTypeAdapter.java:81)
2020-06-22_19:00:26.64220       at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:69)
2020-06-22_19:00:26.64220       at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:125)
2020-06-22_19:00:26.64220       at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:243)
2020-06-22_19:00:26.64221       at com.google.gson.Gson.toJson(Gson.java:669)
2020-06-22_19:00:26.64221       at com.google.gson.Gson.toJson(Gson.java:648)
2020-06-22_19:00:26.64221       at com.google.gson.Gson.toJson(Gson.java:603)
2020-06-22_19:00:26.64221       at com.google.gson.Gson.toJson(Gson.java:583)

41 threads waiting on that lock....

$ grep -c 'waiting to lock <0x00000000d016dc68>' /opt/app/log/current
41

This is with gson-2.8.0

[lyubomyr-shaydariv]

• Thread issues with date formatter #162

• gson/gson/src/main/java/com/google/gson/DefaultDateTypeAdapter.java

  Line 71 in e9ead40

  synchronized (localFormat) {

[sblommers]

We are having the same problem, especcially with heavy load. It simply blocked with a while loop and a concurrent unit test so this surely deserves some attention.

We implemented a custom DateTypeAdapter using FastDateFormat and the troubles went away.

#162

[Marcono1234]

surely deserves some attention

The underlying issue is that Gson currently uses java.text.SimpleDateFormat for the default format, and instances of that class are not thread-safe. Maybe there are ways to improve this though, such as using ThreadLocal (might have its drawbacks with virtual threads?).

However, in general it would be good if you registered a custom TypeAdapter for java.util.Date if possible, because the date format used by the default adapter is problematic, see #2472. In that custom adapter you can then also use the java.time.format.DateTimeFormatter class (converting to Instant temporarily) if you want.

[fireworkit]

meet this error