{"id":1515,"date":"2020-10-03T11:54:02","date_gmt":"2020-10-03T10:54:02","guid":{"rendered":"https:\/\/benjiweber.co.uk\/blog\/?p=1515"},"modified":"2020-10-03T11:54:04","modified_gmt":"2020-10-03T10:54:04","slug":"sealed-java-state-machines","status":"publish","type":"post","link":"https:\/\/benjiweber.co.uk\/blog\/2020\/10\/03\/sealed-java-state-machines\/","title":{"rendered":"Sealed Java State Machines"},"content":{"rendered":"\n

A few years back I posted about how to implement state machines<\/a> that only permit valid transitions at compile time in Java. <\/p>\n\n\n\n

This used interfaces instead of enums, which had a big drawback\u2014you couldn’t guarantee that you know all the states involved. Someone could add another state elsewhere in your codebase by implementing the interface.<\/p>\n\n\n\n

Java 15 brings a preview feature of sealed classes<\/a>. Sealed classes enable us to solve this downside. Now our interface based state machines can not only prevent invalid transitions but also be enumerable like enums.<\/p>\n\n\n\n

If you’re using jdk 15 with preview features enabled you can try out the code<\/a>. This is how it looks to define a state machine with interfaces.<\/p>\n\n\n\n

sealed interface TrafficLight\n       extends State\n       permits Green, SolidAmber, FlashingAmber, Red {}\nstatic final class Green implements TrafficLight, TransitionTo {}\nstatic final class SolidAmber implements TrafficLight, TransitionTo {}\nstatic final class Red implements TrafficLight, TransitionTo {}\nstatic final class FlashingAmber implements TrafficLight, TransitionTo {}<\/pre>\n\n\n\n
The new part is “sealed”<\/em> and “permits”<\/em>. Now it becomes a compile failure to define a new implementation of TrafficLight\u00a0<\/p>\n\n\n\n<\/a>\n\n\n\n
As well as the existing behaviour where it’s a compile time failure to perform a transition that traffic lights do not allow. <\/p>\n\n\n\n<\/a>\n\n\n\n
n.b. you can also skip the compile time checked version and still use the type definitions to runtime check the transitions<\/a>.\u00a0<\/p>\n\n\n\n
Multiple transitions are possible from a state too<\/p>\n\n\n\n
static final class Pending \n  implements OrderStatus, BiTransitionTo {}<\/pre>\n\n\n\n<\/a>\n\n\n\n
Thanks to sealed classes we can also now do enum style enumeration and lookups on our interface based state machines.<\/p>\n\n\n\n
sealed interface OrderStatus\n       extends State\n       permits Pending, CheckingOut, Purchased, Shipped, Cancelled, Failed, Refunded {}\n\n\n@Test public void enumerable() {\n  assertArrayEquals(\n    array(Pending.class, CheckingOut.class, Purchased.class, Shipped.class, Cancelled.class, Failed.class, Refunded.class),\n    State.values(OrderStatus.class)\n  );\n\n  assertEquals(0, new Pending().ordinal());\n  assertEquals(3, new Shipped().ordinal());\n\n  assertEquals(Purchased.class, State.valueOf(OrderStatus.class, \"Purchased\"));\n  assertEquals(Cancelled.class, State.valueOf(OrderStatus.class, \"Cancelled\"));\n}\n<\/pre>\n\n\n\n
These are possible because JEP 360 provides a reflection API<\/a> with which one can enumerate the permitted subclasses of an interface. ( side note the JEP says getPermittedSubclasses() but the implementation seems to use permittedSubclasses<\/a>() )\u00a0
We can add use this to add the above convenience methods to our State interface<\/a> to allow the values<\/a>(), ordinal<\/a>(), and valueOf<\/a>() lookups.<\/p>\n\n\n\n
static > List valuesList(Class stateMachineType) {\n   assertSealed(stateMachineType);\n\n   return Stream.of(stateMachineType.permittedSubclasses())\n       .map(State::classFromDesc)\n       .collect(toList());\n}\n\nstatic > Class valueOf(Class stateMachineType, String name) {\n   assertSealed(stateMachineType);\n\n   return valuesList(stateMachineType)\n       .stream()\n       .filter(c -> Objects.equals(c.getSimpleName(), name))\n       .findFirst()\n       .orElseThrow(IllegalArgumentException::new);\n}\nstatic , U extends T> int ordinal(Class stateMachineType, Class instanceType) {\n   return valuesList(stateMachineType).indexOf(instanceType);\n}<\/pre>\n\n\n\n
There are more details on how the transition checking works and more examples of where this might be useful in the original post<\/a>. Code is on github<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"
A few years back I posted about how to implement state machines that only permit valid transitions at compile time in Java. This used interfaces instead of enums, which had a big drawback\u2014you can’t guarantee that you know all the states involved. Java 15 brings a preview feature of sealed classes, solving this downside. <\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[8],"tags":[],"yoast_head":"\n\n\n\n\n\n\n\n\n\n\n\n\n