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This article mainly explains "what is the strong, soft, virtual and weak citation of JAVA". The content of the explanation in the article is simple and clear, and it is easy to learn and understand. Please follow the editor's train of thought to study and learn "what is the strong, soft, virtual and weak citation of JAVA?"

Strong citation

As long as there is a reference, the target object will not be garbage disposed of by GC

Soft reference

SoftReference wrapper is used. When there is insufficient memory, GC will recycle the corresponding soft reference object.

GC will be specially handled (the call GC,HotSpot JVM shown is stop other threads, and GC; as soon as possible will not GC immediately)

Virtual reference

PhantomReference is used in conjunction with queues (JVM is in use, and GC's underlying implementation is in use)

Help reclaim out-of-heap memory, such as external buffers that netty supports access. JVM points directly to the reference to this buffer, and individual threads scan the wrapped queue for recycling.

Weak reference

If the WeakReference is between a strong reference and a soft reference, the object will be recycled after a GC.

Example 1: ThreadLocal is an embodiment of weak references, and the thread's local Map is inherited from WeakReference

The ThreadLocal Set method sets the current object to the Map of the current thread. Key is the ThreadLocal object and value is the target object.

The following figure is detailed:

Example 2: transaction @ Transaction

Different DAO calling methods, how to ensure access to the same database connection to ensure transaction consistency

When the method that starts execution is executed, the acquired database connection object is put into the ThreadLocalMap of the current thread

The next execution method, which is fetched directly from the ThreadLocalMap of the current thread, guarantees the same database connection to ensure the transaction.

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