什么是循环依赖#

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@Service
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public class A {
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    @Autowired
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    private B b;
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}
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@Service
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public class B {
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    @Autowired
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    private A a;
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}
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//或者自己依赖自己
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@Service
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public class A {
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    @Autowired
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    private A a;
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}