我有一个名为ChildPlugin的子模块,我从主模块注入类如下:
public class ChildPlugin {
private ExampleClass demo;
@Inject
public void setDemo(ExampleClass demo) {
this.demo = demo;
}
}
问题是,我不知道主模块是否绑定了ExampleClass
,如果它不是Guice在创建注入器时抛出异常。我想做的是让Guice传递null
或Optional.empty
,如果ExampleClass没有绑定。
我无法访问主模块,所以我不能将ExampleClass
的粘合剂更改为OptionalBinder
,我尝试了@Nullable
和Optional<ExampleClass>
在ChildPlugin.setDemo
方法中,但它不起作用。
有两种方法可以做到这一点。
可选注入
使用com.google.inject.Inject注释。它允许您在注释上指定optional。请看下面的例子:
public class GuiceInjectOptional extends AbstractModule {
@Override
protected void configure() {
// method 1:
bind(B.class).in(Singleton.class);
}
public static class A {
private String name;
// non null constructor so that A can't be instantiated automatically by guice
public A(String name) {
this.name = name;
}
@Override
public String toString() {
return "I am: " + name;
}
}
public static class B {
@Inject(optional=true)
A obj;
void run() {
System.out.println("Object is: " + obj);
}
}
public static void main(String[] args) {
Injector injector = Guice.createInjector(new GuiceInjectOptional());
injector.getInstance(B.class).run();;
}
}
类B上的注释表明A是可选的。它不会被注射。运行代码片段打印出:
Object is: null
方法2(这是您在指南4+之后执行的方法)。您可以指定可选绑定。如果您愿意,这些绑定甚至允许您定义默认值。然后你可以像我写的这段代码那样注入一个可选的值:
public class GuiceInjectOptional extends AbstractModule {
@Override
protected void configure() {
// set up optional binding for A.
OptionalBinder.newOptionalBinder(binder(), A.class);
bind(B.class).in(Singleton.class);
}
public static class A {
private String name;
// non null constructor so that A can't be instantiated automatically by guice
public A(String name) {
this.name = name;
}
@Override
public String toString() {
return "I am: " + name;
}
}
public static class B {
@Inject
Optional<A> obj;
void run() {
System.out.println("Object is present: " + obj.isPresent());
System.out.println("Object is: " + obj);
}
}
public static void main(String[] args) {
Injector injector = Guice.createInjector(new GuiceInjectOptional());
injector.getInstance(B.class).run();;
}
}
Inject注释现在是非可选的,但是guice知道类A可能被绑定,也可能没有被绑定。运行代码片段将输出:
Object is present: false
Object is: Optional.empty
最后,你可以正常绑定A,然后guice会注入它:
public class GuiceInjectOptional extends AbstractModule {
@Override
protected void configure() {
// set up optional binding for A.
OptionalBinder.newOptionalBinder(binder(), A.class);
bind(A.class).toInstance(new A("Pandaa!"));
bind(B.class).in(Singleton.class);
}
public static class A {
private String name;
// non null constructor so that A can't be instantiated automatically by guice
public A(String name) {
this.name = name;
}
@Override
public String toString() {
return "I am: " + name;
}
}
public static class B {
@Inject
Optional<A> obj;
void run() {
System.out.println("Object is present: " + obj.isPresent());
System.out.println("Object is: " + obj);
}
}
public static void main(String[] args) {
Injector injector = Guice.createInjector(new GuiceInjectOptional());
injector.getInstance(B.class).run();;
}
}
上面会打印:
Object is present: true
Object is: Optional[I am: Pandaa!]
这就是你如何用guice获得故障安全的可选绑定:)我希望这对你有帮助。
编辑:我刚刚看到了guide -3标签,所以你会想要使用可选的注释方法而不是可选的绑定器。使用可选注释,它将保持为空,而不是可选值。
阿图尔