What is a reference type in java

Reference types in Java refer to all types except basic data types. There are four reference types in Java, namely strong reference, soft reference, weak reference and virtual reference. Reference types and primitive types have different characteristics and usage.

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Java provides two different types: reference types and primitive types (or Built-in types)

Reference types and primitive types behave completely differently, and they have different semantics.

Reference types and primitive types have different characteristics and usage, they include: size and speed issues, which type of data structure this type is stored in, when reference types and primitive types

The default value specified when used as instance data for a class.

The default value of object reference instance variables is null, while the default values ​​of primitive type instance variables are related to their type. At the same time, for the consistency of object-oriented operations.

These basic types have corresponding encapsulation types: Integer, Short, Long, Byte, Float, Double, Character, etc.

Java’s 4 reference types:

They are strong reference (StrongReference), soft reference (SoftReference), weak reference (WeakReference) and PhantomReference (virtual reference) ), they were Ranked from most likely to least likely to be GC collected.

StrongReference

As long as the strong reference exists, the garbage collector will never reclaim the referenced object, even if there is insufficient memory, the JVM will directly Throws OutOfMemoryError and will not recycle. like If you want to break the connection between the strong reference and the object, you can explicitly assign the strong reference to null, so that the JVM can recycle the object in a timely manner

Soft Reference (SoftReference)

Soft references are used to describe some non-essential but still useful objects. When there is enough memory, the soft reference object will not be recycled. Only when the memory is insufficient, the system will recycle the soft reference object. Like, if there is still not enough memory after recycling the soft reference object, a memory overflow exception will be thrown. This feature is often used to implement caching technology, such as web page caching, image Caching etc.

After JDK1.2, use the java.lang.ref.SoftReference class to represent soft references.

Weak Reference(WeakReference)

The reference strength of a weak reference is weaker than that of a soft reference. Regardless of whether the memory is sufficient, as long as the JVM starts garbage collection, those that are Objects associated with weak references will be recycled.

After JDK1.2, use java.lang.ref.WeakReference to represent weak references.

PhantomReference(virtual reference)

Virtual reference is the weakest reference relationship. If an object only holds a phantom reference, then it has no reference. Likewise, it may be recycled at any time. After JDK1.2, it is represented by the PhantomReference class. By looking at the source code of this class, we find that it has only one constructor and one get() method, and its get() method is just Returning a null means that the object will never be obtained through a virtual reference. Virtual references must be used together with the ReferenceQueue reference queue.

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