Commands get instantiated multiple times per run

[ctrueden]

As discussed in this Zulip thread, when invoking a SciJava Command plugin, multiple instances of the command might get instantiated and then thrown away, just to retrieve default parameter values. This can create confusion if there is actual business logic that gets executed during object construction, because it will then be executed multiple times, and if there are side effects, and/or performance implications, they will occur repeatedly.

Stack trace illustrating the multiple instantiations

Breakpoint reached at sc.iview.commands.add.AddBox.<init>(AddBox.kt:75)
Breakpoint reached
    at sc.iview.commands.add.AddBox.<init>(AddBox.kt:75)
    at java.lang.invoke.DirectMethodHandle$Holder.newInvokeSpecial(DirectMethodHandle$Holder:-1)
    at java.lang.invoke.Invokers$Holder.invokeExact_MT(Invokers$Holder:-1)
    at jdk.internal.reflect.DirectConstructorHandleAccessor.invokeImpl(DirectConstructorHandleAccessor.java:86)
    at jdk.internal.reflect.DirectConstructorHandleAccessor.newInstance(DirectConstructorHandleAccessor.java:62)
    at java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:502)
    at java.lang.reflect.ReflectAccess.newInstance(ReflectAccess.java:128)
    at jdk.internal.reflect.ReflectionFactory.newInstance(ReflectionFactory.java:304)
    at java.lang.Class.newInstance(Class.java:725)
    at org.scijava.plugin.PluginInfo.createInstance(PluginInfo.java:304)
    at org.scijava.command.CommandInfo.createInstance(CommandInfo.java:248)
    at org.scijava.command.CommandModule.instantiateCommand(CommandModule.java:248)
    at org.scijava.command.CommandModule.<init>(CommandModule.java:98)
    at org.scijava.command.CommandInfo.createModule(CommandInfo.java:326)
    at org.scijava.module.DefaultModuleService.createModule(DefaultModuleService.java:167)
    at org.scijava.module.DefaultModuleService.run(DefaultModuleService.java:206)
    at org.scijava.module.DefaultModuleService.run(DefaultModuleService.java:197)
    at org.scijava.module.DefaultModuleService.run(DefaultModuleService.java:182)
    at org.scijava.menu.ShadowMenu.run(ShadowMenu.java:335)
    at org.scijava.ui.swing.menu.AbstractSwingMenuCreator$1.actionPerformed(AbstractSwingMenuCreator.java:169)
...
now have 1 CountyThingies
Breakpoint reached at sc.iview.commands.add.AddBox.<init>(AddBox.kt:75)
Breakpoint reached
    at sc.iview.commands.add.AddBox.<init>(AddBox.kt:75)
    at java.lang.invoke.DirectMethodHandle$Holder.newInvokeSpecial(DirectMethodHandle$Holder:-1)
    at java.lang.invoke.Invokers$Holder.invokeExact_MT(Invokers$Holder:-1)
    at jdk.internal.reflect.DirectConstructorHandleAccessor.invokeImpl(DirectConstructorHandleAccessor.java:86)
    at jdk.internal.reflect.DirectConstructorHandleAccessor.newInstance(DirectConstructorHandleAccessor.java:62)
    at java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:502)
    at java.lang.reflect.ReflectAccess.newInstance(ReflectAccess.java:128)
    at jdk.internal.reflect.ReflectionFactory.newInstance(ReflectionFactory.java:304)
    at java.lang.Class.newInstance(Class.java:725)
    at org.scijava.command.CommandModuleItem.getDefaultValue(CommandModuleItem.java:168)
    at org.scijava.module.DefaultModuleService.getDefaultValue(DefaultModuleService.java:321)
    at org.scijava.module.process.DefaultValuePreprocessor.assignDefaultValue(DefaultValuePreprocessor.java:76)
    at org.scijava.module.process.DefaultValuePreprocessor.process(DefaultValuePreprocessor.java:61)
    at org.scijava.module.ModuleRunner.preProcess(ModuleRunner.java:103)
    at org.scijava.module.ModuleRunner.run(ModuleRunner.java:154)
    at org.scijava.module.ModuleRunner.call(ModuleRunner.java:125)
    at org.scijava.module.ModuleRunner.call(ModuleRunner.java:64)
    at org.scijava.thread.DefaultThreadService.lambda$wrap$2(DefaultThreadService.java:247)
    at org.scijava.thread.DefaultThreadService$$Lambda/0x0000000800f2ac68.call(Unknown Source:-1)
    at java.util.concurrent.FutureTask.run$$$capture(FutureTask.java:317)
    at java.util.concurrent.FutureTask.run(FutureTask.java:-1)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
    at java.lang.Thread.runWith(Thread.java:1596)
    at java.lang.Thread.run(Thread.java:1583)
now have 2 CountyThingies
Breakpoint reached at sc.iview.commands.add.AddBox.<init>(AddBox.kt:75)
Breakpoint reached
    at sc.iview.commands.add.AddBox.<init>(AddBox.kt:75)
    at java.lang.invoke.DirectMethodHandle$Holder.newInvokeSpecial(DirectMethodHandle$Holder:-1)
    at java.lang.invoke.Invokers$Holder.invokeExact_MT(Invokers$Holder:-1)
    at jdk.internal.reflect.DirectConstructorHandleAccessor.invokeImpl(DirectConstructorHandleAccessor.java:86)
    at jdk.internal.reflect.DirectConstructorHandleAccessor.newInstance(DirectConstructorHandleAccessor.java:62)
    at java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:502)
    at java.lang.reflect.ReflectAccess.newInstance(ReflectAccess.java:128)
    at jdk.internal.reflect.ReflectionFactory.newInstance(ReflectionFactory.java:304)
    at java.lang.Class.newInstance(Class.java:725)
    at org.scijava.command.CommandModuleItem.getDefaultValue(CommandModuleItem.java:168)
    at org.scijava.module.DefaultModuleService.getDefaultValue(DefaultModuleService.java:321)
    at org.scijava.module.process.DefaultValuePreprocessor.assignDefaultValue(DefaultValuePreprocessor.java:76)
    at org.scijava.module.process.DefaultValuePreprocessor.process(DefaultValuePreprocessor.java:61)
    at org.scijava.module.ModuleRunner.preProcess(ModuleRunner.java:103)
    at org.scijava.module.ModuleRunner.run(ModuleRunner.java:154)
    at org.scijava.module.ModuleRunner.call(ModuleRunner.java:125)
    at org.scijava.module.ModuleRunner.call(ModuleRunner.java:64)
    at org.scijava.thread.DefaultThreadService.lambda$wrap$2(DefaultThreadService.java:247)
    at org.scijava.thread.DefaultThreadService$$Lambda/0x0000000800f2ac68.call(Unknown Source:-1)
    at java.util.concurrent.FutureTask.run$$$capture(FutureTask.java:317)
    at java.util.concurrent.FutureTask.run(FutureTask.java:-1)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
    at java.lang.Thread.runWith(Thread.java:1596)
    at java.lang.Thread.run(Thread.java:1583)
now have 3 CountyThingies
Breakpoint reached at sc.iview.commands.add.AddBox.<init>(AddBox.kt:75)
Breakpoint reached
    at sc.iview.commands.add.AddBox.<init>(AddBox.kt:75)
    at java.lang.invoke.DirectMethodHandle$Holder.newInvokeSpecial(DirectMethodHandle$Holder:-1)
    at java.lang.invoke.Invokers$Holder.invokeExact_MT(Invokers$Holder:-1)
    at jdk.internal.reflect.DirectConstructorHandleAccessor.invokeImpl(DirectConstructorHandleAccessor.java:86)
    at jdk.internal.reflect.DirectConstructorHandleAccessor.newInstance(DirectConstructorHandleAccessor.java:62)
    at java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:502)
    at java.lang.reflect.ReflectAccess.newInstance(ReflectAccess.java:128)
    at jdk.internal.reflect.ReflectionFactory.newInstance(ReflectionFactory.java:304)
    at java.lang.Class.newInstance(Class.java:725)
    at org.scijava.command.CommandModuleItem.getDefaultValue(CommandModuleItem.java:168)
    at org.scijava.module.DefaultModuleService.getDefaultValue(DefaultModuleService.java:321)
    at org.scijava.module.process.DefaultValuePreprocessor.assignDefaultValue(DefaultValuePreprocessor.java:76)
    at org.scijava.module.process.DefaultValuePreprocessor.process(DefaultValuePreprocessor.java:61)
    at org.scijava.module.ModuleRunner.preProcess(ModuleRunner.java:103)
    at org.scijava.module.ModuleRunner.run(ModuleRunner.java:154)
    at org.scijava.module.ModuleRunner.call(ModuleRunner.java:125)
    at org.scijava.module.ModuleRunner.call(ModuleRunner.java:64)
    at org.scijava.thread.DefaultThreadService.lambda$wrap$2(DefaultThreadService.java:247)
    at org.scijava.thread.DefaultThreadService$$Lambda/0x0000000800f2ac68.call(Unknown Source:-1)
    at java.util.concurrent.FutureTask.run$$$capture(FutureTask.java:317)
    at java.util.concurrent.FutureTask.run(FutureTask.java:-1)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
    at java.lang.Thread.runWith(Thread.java:1596)
    at java.lang.Thread.run(Thread.java:1583)
now have 4 CountyThingies

One idea to fix this would be to infer all of a command's default values the first time an instance is created as a module. Because an instance of the command is instantiated immediately: the one that is actually going to get run. The later ones are just throwaways. But if upon first instantiation, we inspected the instance's field values immediately and cached them into the CommandInfo itself, then we'd be able to reuse those values from the cache whenever getDefaultValue() is subsequently called, without instantiating throwaway instances.