问题
I want do a binding like this,
bind(Supplier<TestClass>).toProvider(Provider<Supplier<TestClass>>).in(Singleton.class);
The provider is returned by an external function so, inside the toProvider()
, I call that function and it returns Provider<Supplier<TestClass>>
.
The supplier is from guava, the reason to do this kind of thing is, there is a file associated with the TestClass and I need to read that file and assign those values to the respective fields of TestClass.
And that file change at run time, So I need a way to refresh the values stored in TestClass. To-Do that guava supplier I used. Guava supplier has a get method and when that get method is called, if I used memoizeWithExpiration()
to create the instance then it checks the TTL value and if it passed, then I can specify a lambda function to read the file and assign values.
So I need to inject Supplier<TestClass>
like this
@Inject
Supplier<TestClass> someClassSupplier;
But For doing that binding with Guice is confusing for me.
回答1:
You can use the following kind of code to do what you want:
class ServiceModule extends AbstractModule {
private TestClass readTestClassFromFile() {
return new TestClass();
}
// Cache an instance for 5 seconds.
private final Supplier<TestClass> testClassSupplier = Suppliers.memoizeWithExpiration(this::readTestClassFromFile, 5, SECONDS);
@Provides TestClass provideTestClass() { // Don't declare as singleton
return testClassSupplier.get();
}
}
Then, in your class:
class Service {
@Inject
Provider<TestClass> testClassProvider; // Inject the provider, not the instance itself, or any supplier.
void doSomething() throws Exception {
TestClass a = testClassProvider.get();
TestClass b = testClassProvider.get();
Thread.sleep(6000); // Sleep for 6 seconds
TestClass c = testClassProvider.get();
System.out.println(a == b); // Prints true
System.out.println(a == c); // Prints false
}
}
You requested for a generic way of doing this, so here it is, check the bindSupplier
method:
import static com.google.common.base.Suppliers.memoizeWithExpiration;
import com.google.inject.*;
import java.util.concurrent.TimeUnit;
import java.util.function.Supplier;
public class Main {
public static void main(String[] args) throws Exception {
Guice.createInjector(new ServiceModule())
.getInstance(Service.class)
.doSomething();
}
static class ServiceModule extends AbstractModule {
Dependency createDependency() { return new Dependency(); }
// For Java 8+
private <T> void bindSupplier(Class<T> type, Supplier<? extends T> supplier) {
// Definitely avoid .in(Singleton.class) because you want the scope to be defined by the Supplier.
bind(type).toProvider(supplier::get);
}
// For Java 7 and less
// private <T> void bindSupplier(Class<T> type, final Supplier<? extends T> supplier) {
// bind(type).toProvider(new Provider<T>() {
// @Override public T get() { return supplier.get(); }
// });
// }
@Override protected void configure() {
bindSupplier(Dependency.class,
memoizeWithExpiration(this::createDependency, 3, TimeUnit.SECONDS));
}
}
static class Dependency {}
static class Service {
@Inject Provider<Dependency> dependencyProvider;
void doSomething() throws InterruptedException {
Dependency a = dependencyProvider.get();
Dependency b = dependencyProvider.get();
Thread.sleep(4000);
Dependency c = dependencyProvider.get();
System.out.printf("a == b ? %s%n", a == b); // true
System.out.printf("a == c ? %s%n", a == c); // false
}
}
}
来源:https://stackoverflow.com/questions/65317178/binding-a-guava-supplier-using-guice