The Java object life cycle goes through three stages: creation, use and recycling. 1. Create: trigger class loading through new keywords, the JVM allocates memory in the heap and executes constructor to initialize the object; 2. Use: The object is accessed by reference, and frequently creates large objects can be considered for object pool optimization performance; 3. Recycling: When the object is unreachable, it is recycled by GC. Common algorithms include mark-clearing, copying, mark-collation, and different recyclers adapt to different scenarios; determine whether the object can be recycled dependent reference accessibility, such as explicit null, end of scope, weak reference failure, etc.; methods to avoid memory leaks include reducing global variables, timely logging out listeners, using memory analysis tools, and paying attention to the problem of internal classes holding external class references. Understanding these mechanisms can help improve program performance and stability.

The life cycle and memory management of Java objects are the basis for understanding the running mechanism of Java programs. Many people only focus on functional implementation when writing code, but if they do not understand how objects are created, used, and recycled, it is easy to write programs with memory leaks or poor performance.

How are objects created?

The life cycle of an object in Java starts with the new keyword. For example, if you write Person p = new Person(); , the JVM will allocate a piece of space in the heap memory to store the data of this object and return a reference to the variable p.

Key details:

• Class loading: When a class is first used, the JVM will load it and prepare the constructor.
• Memory allocation: Allocate space for objects in the heap, and the specific size depends on the number and type of fields of the object.
• Constructor execution: Initialize the state of the object, such as setting the default value or passing parameters through the constructor.

This process may seem simple, but if you frequently create large objects (such as large arrays), it will affect performance. So sometimes we will use object pooling technology to reuse objects to reduce the overhead caused by frequent creation and destruction.

How does garbage recycling work?

When an object is no longer referenced by any variable, it becomes "garbage" and is waiting to be recycled by GC (Garbage Collector). Java's GC mechanism automatically manages memory, but it does not mean that we can ignore it at all.

Several common GC algorithms:

• Mark-Sweep: Mark all reachable objects, and the rest is recyclable.
• Copying: Copy the surviving object to another area and then clear the original area.
• Mark-Compact: First mark, then sort the surviving objects to one end, and finally clean up the memory outside the boundary.

Different garbage collectors are suitable for different scenarios, such as Serial is suitable for single-threaded environments, and G1 is suitable for large-scale memory applications. If your application is delay sensitive, you may need to tune the GC parameters, such as adjusting the ratio of young generations to older generations.

When will the object be recycled?

This problem is critical because many memory leaks are here. The criterion for determining whether an object is recyclable is "whether it can still be accessed".

Common unreachable situations:

• Explicitly set to null: for example obj = null;
• Method execution ends: After local variables are out of scope, the reference is naturally invalid.
• Other reference types besides strong references: For example, the keys in WeakHashMap, once there is no strong reference pointing to them, the GC will be recycled next time.

An example that is easy to ignore is cache. If the cache keeps adding data to it without cleaning the policy, it is easy to cause memory overflow. At this time, you can consider using weak references or regular cleaning mechanisms.

How to avoid memory leaks?

Although Java has GC, memory leaks can still occur. The most common situation is that an object is clearly no longer used, but it is still referenced in some places, making it impossible to recycle.

Several practical suggestions:

• Avoid unnecessary global variables, especially static collection classes.
• Remember to log out after registering a listener or callback, such as event listeners.
• Use memory analysis tools (such as VisualVM, MAT) to view memory snapshots and find the paths of unfreed objects.
• Pay attention to the problem that inner classes hold external class references, such as non-static inner classes may implicitly hold external class instances.

For example, if an anonymous inner class runs for a long time (such as a timed task) and holds references to the external class, it may cause the entire external class to be recycled.

Basically that's it. Java's object life cycle and memory management don't seem complicated, but if you don't pay attention to it in actual development, you may fall into a trap. Especially in scenarios where concurrent and large-scale data processing are processed, you need to pay more attention to the creation and recycling methods of objects.

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