Support @Nullable annotations as indicators for optional injection points [SPR-15028]

[spring-projects-issues]

Juergen Hoeller opened SPR-15028 and commented

Along the lines of Guice (https://github.com/google/guice/wiki/UseNullable), we can easily detect @Nullable annotations and automatically mark method parameters and dependency descriptors as optional, i.e. not required, injecting null instead of throwing an UnsatisfiedDependencyException. This is particularly useful with JSR-330's @Inject where not @Autowired-style required attribute is available at the injection annotation level, as an alternative to using Java 8's java.util.Optional (which Spring supports for a while already). This also nicely goes along with our support for nullable type declarations in Kotlin.

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Issue Links:

• Support Kotlin nullable information for @Autowired and @Inject [SPR-14951] #19518 Support Kotlin nullable information for @Autowired and @Inject
• Support for java.util.Optional with @Inject (a la @Autowired's required=false) [SPR-11833] #16452 Support for java.util.Optional with @Inject (a la @Autowired's required=false)

Referenced from: commits 12aa14d