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Exception in thread "main" java.lang.ClassCastException: java.base/jdk.internal.loader.ClassLoaders$AppClassLoader cannot be cast to java.base/java.net.URLClassLoader at org.springframework.boot.devtools.restart.DefaultRestartInitializer.getUrls(DefaultRestartInitializer.java:93) at org.springframework.boot.devtools.restart.DefaultRestartInitializer.getInitialUrls(DefaultRestartInitializer.java:56) at org.springframework.boot.devtools.restart.Restarter.(Restarter.java:138) at org.springframework.boot.devtools.restart.Restarter.initialize(Restarter.java:537) at org.springframework.boot.devtools.restart.RestartApplicationListener.onApplicationStartedEvent(RestartApplicationListener.java:68) at org.springframework.boot.devtools.restart.RestartApplicationListener.onApplicationEvent(RestartApplicationListener.java:45) at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:166) at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:138) at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:121) at org.springframework.boot.context.event.EventPublishingRunListener.started(EventPublishingRunListener.java:63) at org.springframework.boot.SpringApplicationRunListeners.started(SpringApplicationRunListeners.java:48) at org.springframework.boot.SpringApplication.run(SpringApplication.java:304) at org.springframework.boot.SpringApplication.run(SpringApplication.java:1186) at org.springframework.boot.SpringApplication.run(SpringApplication.java:1175) at com.rme.hub.RmeApplication.main(RmeApplication.java:24)
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ClassCastException is a runtime exception raised in Java when we try to improperly cast a class from one type to another. It's thrown to indicate that the code has attempted to cast an object to a related class, but of which it is not an instance.
// type cast an parent type to its child type. In order to deal with ClassCastException be careful that when you're trying to typecast an object of a class into another class ensure that the new type belongs to one of its parent classes or do not try to typecast a parent object to its child type.
You would get a ClassCastException, but not whithin the cast-method, but at the assignment to s. But this will still lead to an error later, when the you use your class.
ClassCastException is one of the unchecked exception in Java. It can occur in our program when we tried to convert an object of one class type into an object of another class type.
Judging by the presence of java.base/jdk.internal.loader.ClassLoaders in the stack trace, you are using Java 9 or later. Spring Boot's DefaultRestartInitializer is trying to cast the app class loader to a URLClassLoader. This works in Java 8 and earlier but does not work with Java 9 or later. Spring Boot had been updated in 2.0 to cope with this change in Java 9.
If you want to use Spring Boot with Java 9 or later, you should upgrade to Spring Boot 2. At the time of writing, the latest release is 2.0.5 which supports Java 8, 9, and 10. Spring Boot 2.1, which will be released later this year, will add support for Java 11.
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