What are the mechanisms and strategies of Java memory management?

Java adopts an automatic memory management mechanism and uses a mark and clear algorithm through the garbage collector to recycle objects that are no longer used. The main strategies include Serial GC, Parallel GC, CMS GC and G1 GC. In practical cases, weak references can manage temporary objects, and soft references can cache objects.

Java memory management mechanism and strategy

Java adopts an automatic memory management mechanism, which can automatically recycle items that are no longer needed. objects to avoid memory leaks and programming errors. The main principles are as follows:

1. Object allocation

When a new object is created, the Java Virtual Machine (JVM) will automatically allocate the required space. Heap memory is a data area dedicated to storing objects.

// 创建一个新对象
Person person = new Person();

// JVM 将 person 对象分配到堆内存中
// ...

2. Garbage collection

The JVM will run the garbage collector regularly and automatically recycle objects that are no longer used. The garbage collector uses the mark and clear algorithm:

• Marking phase: The garbage collector will traverse all reachable objects (starting from the root reference) and mark the reachable objects.
• Cleaning phase: The garbage collector will traverse the entire heap memory and clear unmarked objects (that is, objects that are no longer reachable).

3. Reference

References between objects define the reachability of the object, which is the basis for the garbage collector to determine whether the object needs to be recycled:

Strong reference: The only reference pointing to the object. The object under the strong reference will never be recycled.
Weak reference: When an object is pointed to by a weak reference, if the object no longer has a strong reference, it will be recycled.
Soft reference: When an object is pointed to by a soft reference, it will be recycled when the JVM encounters insufficient memory.
Virtual reference: When an object is pointed to by a virtual reference, the JVM does not guarantee that the object will not be recycled.

4. Memory management strategies

Java memory management provides a variety of strategies to affect the performance of garbage collection:

Serial garbage Recycling (Serial GC): Single-threaded garbage collection, low resource consumption, but low throughput.
Parallel garbage collection (Parallel GC): Using multiple threads to perform garbage collection has high throughput, but high resource consumption.
Concurrent Garbage Collection (Concurrent Mark and Sweep GC): The garbage collector runs concurrently with the application to reduce application pause time.
G1 Garbage Collection (G1 GC): Adopts a generational collection algorithm to divide the heap memory into different areas, resulting in high recycling efficiency.

5. Practical case

Use weak references to manage temporary objects:

// 创建一个弱引用
WeakReference<Person> weakPerson = new WeakReference<>(person);

// ...

// 当 person 对象不再需要时，清除弱引用
weakPerson.clear();

In the above example, when the person object It will be garbage collected when it is no longer pointed to by a strong reference, even if weakPerson still points to it.

Use soft references to cache objects:

// 创建一个软引用
SoftReference<Person> softPerson = new SoftReference<>(person);

// ...

// 当 JVM 内存不足时，软引用对象将被回收
if (softPerson.get() == null) {
    // 重新创建 person 对象
}

In the above example, when the JVM runs out of memory, the softPerson reference object will be recycled, but the person object will still be on the heap in memory until it is no longer pointed to by a strong reference.

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