Garbage collection mechanism in java

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1 garbage collection

1.1 For objects in java's gc heap, when does the object change from the state to be recycled to the active state (garbage becomes a non-garbage object)

sure. First, try-with-resourcesyou need a class that implements AutoCloseablethe or Closeableinterface in order to use it. Both interfaces have one method that needs to be implemented: close().

Here is a simple example:

1. Define a AutoCloseableclass that implements :

public class MyResource implements AutoCloseable {
    
    

    public MyResource() {
    
    
        System.out.println("Resource opened.");
    }

    public void use() {
    
    
        System.out.println("Resource is being used.");
    }

    @Override
    public void close() {
    
    
        System.out.println("Resource closed.");
    }
}

2. Use try-with-resourcesthe structure :

public class TryWithResourcesDemo {
    
    

    public static void main(String[] args) {
    
    
        try (MyResource resource = new MyResource()) {
    
    
            resource.use();
        } catch (Exception e) {
    
    
            System.out.println("An exception occurred: " + e.getMessage());
        }
    }
}

When you run the above code, you will see the following output:

Resource opened.
Resource is being used.
Resource closed.

In try-with-resourcesthe struct, the resource resource's close()method is automatically trycalled at the end of the block, ensuring that the resource is properly closed, even if an exception is thrown while using the resource.

The main advantage of this structure is that it ensures proper and timely shutdown of resources, thereby reducing resource leaks and related errors.

1.2 Show me a demo of resurrecting objects using interfaces such as try-with-resources and AutoCloseable

In try-with-resourcesthe context of , resurrecting objects is uncommon and not recommended. This is because the main purpose of this mechanism is to ensure that resources are properly closed, not to manage the lifecycle of objects.

But to illustrate, close()inside a method, you can try to resurrect the object, although it's not a good practice to do so.

Suppose we have one similar to the previous MyResource, but this time it has a static reference to the resurrected object:

public class ResurrectingResource implements AutoCloseable {
    
    
    public static ResurrectingResource instance;

    public ResurrectingResource() {
    
    
        System.out.println("Resource opened.");
    }

    public void use() {
    
    
        System.out.println("Resource is being used.");
    }

    @Override
    public void close() {
    
    
        System.out.println("Resource closed.");
        instance = this;  // 尝试复活对象
    }
}

You can use this resource with try-with-resources:

public class ResurrectionDemo {
    
    

    public static void main(String[] args) {
    
    
        try (ResurrectingResource resource = new ResurrectingResource()) {
    
    
            resource.use();
        }
        
        // 检查对象是否已复活
        if (ResurrectingResource.instance != null) {
    
    
            System.out.println("Resource was resurrected!");
            ResurrectingResource.instance.use();
        } else {
    
    
            System.out.println("Resource was not resurrected.");
        }
    }
}

When you run this code, you will see:

Resource opened.
Resource is being used.
Resource closed.
Resource was resurrected!
Resource is being used.

While this does show how to close()resurrect an object within a method, it's important to emphasize that this is not a recommended or common use of try-with-resourcesor AutoCloseable. In practical application design, the resurrected object situation should be avoided, because it may introduce unexpected behavior and difficult to debug problems.

1.3 So it is rare for garbage in the java heap to become non-garbage?

Yes, it is rare for an object in the Java heap that is considered garbage to become a non-garbage object (also known as object resurrection), and usually because of some behavior in the finalize method. As mentioned earlier, the case of object resurrection is not recommended, as it may introduce complexity and unpredictable behavior. Most modern Java programs or libraries rarely rely on the finalize method, and may avoid using it entirely.

1.4 If a method creates an object at the beginning and is not used until the end of the method, will the object be recycled when the method is executed?

If a method creates an object at the beginning, but does not use it until the end of the method, the object will not be garbage collected during the method's execution. Java's garbage collector uses a reachability analysis algorithm to determine which objects can be recycled. Simply put, starting from GC Roots (such as local variables on the stack, static variables, JNI references, etc.), an object is not considered garbage if it is reachable at analysis time.

In the situation you describe, although the object is not used for most of the method, it is still reachable in a local variable on the stack, so it is not considered garbage and will not be executed in the method is recycled. Only when the local variable goes out of scope after the method ends, the object may be considered garbage (provided there are no other references to it).

1.5 The object created in a method will not be recycled immediately after execution, but will be marked as garbage after the method is executed, right?

An object created within a method may be marked as garbage after the method is executed, but it will not be collected immediately. The timing of object recycling is determined by the strategy and trigger conditions of the garbage collector. For example, objects may still exist in the heap until the next GC cycle starts. The specific timing of recycling depends on the type and configuration of the garbage collector (for example, using Serial, Parallel, CMS, G1, etc.) and the triggering conditions of garbage collection (such as the memory usage threshold).

1.6 Only when the local variable goes out of scope after the method ends, the object may be regarded as garbage. The local variable out of scope means that the reference on the stack frame no longer exists and is replaced by another stack frame, so the new GC roots cannot be searched when traversing downwards, will they be recycled in the end?

Yes, when the method ends, the stack frame of the method will be popped, so the local variables on it will also go out of scope. This means that these local variables (if they are object references) are no longer considered GC Roots. Therefore, if these local variables are the only references to some objects, then those objects will become unreachable. In the next garbage collection cycle, the garbage collector will check the reachability of the object, and the unreachable object will be marked as garbage, and will be cleaned up in the subsequent recycling phase.

As you said, when new GC Roots start traversing, if an object cannot be reached (i.e. it is unreachable), then the object is marked and recycled in a later stage.

1.7 Isn't a dead object a kind of unreachable object, why can it be referenced?

Answer: In the remarking phase of cms, gc roots will be rescanned. These gc roots exist in each thread stack frame. If a thread revives the previous objects to be recycled during the concurrent marking phase, then in the remarking phase this phase will be Scan the gc roots of this newly opened thread, and traverse down through the gc roots, you will find that the dead object has become alive again.

You spoke about the details of the CMS (Concurrent Mark-Sweep) garbage collector in Java. In the description you provided, you discussed the phenomenon of object resurrection that can occur during the CMS relabeling phase.

Your description is mostly correct, but I'm going to add some simple clarifications to your description to make it clearer.

1. Dead Objects vs. Unreachable Objects : First of all, dead objects are indeed those objects that are no longer referenced, also known as unreachable objects. Unreachable means that there is no reference path from GC Roots (such as thread stack, static variables, etc.) to these objects.

2. Object resurrection : In Java, objects can be "resurrected" under certain circumstances. Specifically, when an object's finalize()method is invoked, that object has a chance to be re-referenced, thus bringing it back to life. Therefore, after the CMS concurrent mark phase, if some objects' finalize()methods are executed and re-referenced, these objects may be considered as live objects in the remark phase.

3. CMS's remark phase : The remark phase you mentioned is indeed a phase in CMS to ensure that objects that might have been missed or whose state had changed during the concurrent mark phase are properly handled. At this stage, GC will start marking reachable objects from GC Roots again. Therefore, if there are objects resurrected from the "dead" state during this time, they will be correctly marked as live objects.

To sum up, your description is correct, it is indeed possible for objects to be resurrected after the CMS's concurrent mark phase, and during the remark phase, these resurrected objects are correctly marked as live objects.

1.9 How dead objects are referenced:

Under normal circumstances, a dead object (that is, an unreachable object) will no longer be referenced by the program. But in the object's finalize method, it may assign itself to a surviving static variable or other reachable variable, so that the object will become reachable again, thus achieving "resurrection".

Such "object resurrection" cases are rare, but certainly possible. This is also why in Java, an object's finalize method will only be called once, even if it becomes unreachable again. This is to prevent the object from being resurrected through the finalize method every time it is considered garbage, resulting in never being recycled.

Because of this potential problem with the finalize method and some other performance considerations, its use is generally deprecated and marked as obsolete after Java 9.

1.10 Will CMS recycle objects that were originally alive but died later?

The answer is yes. But this requires some explanation.

Objects may still be alive (reachable) during the concurrent mark phase of the CMS. But in subsequent phases, such as between concurrent marking and remarking, some objects may become dead (unreachable). **CMS's remarking phase (Final Remark) will handle changes that may occur during concurrent marking and capture these new dead objects. ** Therefore, when the CMS enters the concurrent sweep phase, these objects that died after the concurrent mark will be recycled.

Therefore, CMS can indeed handle and recycle objects that become unreachable after concurrent marking.

1.11 Is the card table mark of cms in the area of ​​the referenced object? For example, the young generation points to the old generation, whether it is the card table marking the young generation or the old generation

In HotSpot JVM, when a cross-generational reference occurs (such as a young generation object pointing to an old generation object), this is tracked by marking the area where the object is written (that is, the area where the reference is modified) in the card table. Specifically, if a young generation object references an old generation object, the card table area where the young generation object is located will be marked as "dirty". Therefore, the answer is: mark on the card table of the young generation.

1.12 Why does CMS need to reserve space:

CMS (Concurrent Mark-Sweep) is a collector mainly for the old generation. Since it is designed to minimize application pause times, the CMS allows application threads to run concurrently during most phases. But this design also brings a problem: between the CMS cleaning phase and the next GC, the application may continue to allocate new objects. If the space in the old generation is completely used up, it will cause a full GC, so the goal of CMS - to reduce pauses - will be defeated.

In order to avoid this situation, CMS designed a "reserve space" mechanism. When the usage of the old generation exceeds a certain threshold, CMS will trigger GC in advance instead of waiting until the old generation is completely used up. This way, it ensures that there is enough space to handle new object allocations during the concurrent phase.

The setting of this reserved space is controlled by the JVM parameter -XX:CMSInitiatingOccupancyFraction. The default value is 68, which means that when the usage rate of the old age exceeds 68%, the CMS collection will be started.

1.13 Specific things to be done in the concurrent preprocessing of CMS

1.14 Specific things to do in CMS relabeling

2 Resource release

2.1 The relationship between resource release and garbage collection

1. Resource release :

   • Refers to the timely release of external resources held by the object after use, such as file handles, network connections, database connections, etc.
   • In Java, resources can be released by implementing the Closeableor AutoCloseableinterface and overriding the method. closeUse try-with-resourcesthe statement to ensure that resources are properly released.
   • Resource release is the developer's responsibility. If not handled properly, it can lead to resource leaks, which can cause various problems (such as the file cannot be opened because the file handle is exhausted).

2. garbage collection :

   • Garbage collection is an automatic process whose purpose is to find those objects that are no longer used and reclaim their memory.
   • In Java, garbage collection is handled by the garbage collector (Garbage Collector, GC). The GC runs periodically to mark and reclaim unreachable objects.
   • Developers usually do not need to directly manage memory or manually release objects, but understanding how this works can help to write more efficient code.

The relationship between them :

• Resource release focuses on external resources, while garbage collection focuses on internal memory resources.
• Resource release should be done as early as possible to ensure timely release of system resources. But whether an object's memory is reclaimed is determined by the GC, which may happen at any point after the resource is released.
• Improper resource release may lead to resource leaks, although the object's memory may eventually be reclaimed by GC. But conversely, even if the object's external resources have been properly released, if the object is still reachable, its memory will not be reclaimed by GC.

In summary, both resource release and garbage collection are designed to ensure efficient management of resources. Resource release ensures that external resources are released, while garbage collection ensures that memory that is no longer in use is reclaimed.

2.2 Assuming that the try statement is in the method, if the try is executed but the method is not executed, will it be recycled?

try (BufferedReader br = new BufferedReader(new FileReader("filename.txt"))) {
    
    
    // ... read from the file
} 

In the above try-with-resourcesstatement, BufferedReaderthe instance will automatically call its method after the code block ends brto release the resources it occupies. But this is different from the behavior of Java garbage collection.tryclose

Regarding your question, the specific answer is as follows:

1. Resource release : tryAfter the code block ends, because the or interface BufferedReaderis implemented , its method will be called automatically to close the file resource. This has nothing to do with whether the object is recycled, but related resources (such as file handles) are released in due course.CloseableAutoCloseableclosebr

2. Object recycling : Although the resources have been released, brthe memory occupied by the object itself in the heap is not immediately reclaimed. brWhen an object is garbage collected depends on the behavior of the garbage collector. After try-with-resourcesthe statement ends, brthe reference to goes out of scope, making brthe object no longer reachable, so it can be marked as garbage, pending subsequent garbage collection.

Summary: tryAfter the statement ends, BufferedReaderthe closemethod ensures that the file resource is released, and brwhen the object is reclaimed depends on the garbage collector.

2.3 try-with-resource and try-catch/throws exception Throwing exceptions are two completely different concepts, although their keywords overlap

2.3.1 Will try-with-resources catch exceptions?

Answer: No,
if the try-with-resource statement is used in a method, and this statement may throw an exception, such as FileNotFoundException, that is to say, this statement not only applies for resources but also throws an exception; then in the end, you still have to Add a catch to it, or add throws Exception to the method signature,
as shown below: The try here is reused, which is both the try of try-wth-resouce and the try of try-catch

    public void method(String[] args) {
    
    
        try (BufferedReader br = new BufferedReader(new FileReader("filename.txt"))) {
    
    
            // ... read from the file
        } catch (FileNotFoundException e) {
    
    
            e.printStackTrace();
        } catch (IOException e) {
    
    
            e.printStackTrace();
        }
    }

2.3.2 Must try-with-resource and try-catch be used at the same time?

Answer: Not necessarily. It depends on whether the logic of the resource statement itself may throw an exception. For example, sc does not require us to actively throw an exception.

void sc(){
    
    
    try (Scanner scanner = new Scanner(System.in)) {
    
    
		
	}

}

2.3.3 If an exception is thrown or try-catch catches an exception in the method, will the resource be closed correctly?

If the try-with-resource statement is not used in the statement to open the resource or the close method is not displayed to close the resource, the resource will not be closed even if an exception is thrown, because they are two independent concepts.

If you use in the method signature to throwsdeclare the exceptions that may be thrown, and you use in the method body try-with-resourcesto manage resources, then even if the method throws an exception, try-with-resourcesit will still ensure that the resources are properly closed.

For example:

public void readFile() throws IOException {
    
    
    try (BufferedReader br = new BufferedReader(new FileReader("filename.txt"))) {
    
    
        // ... read from the file, which might throw IOException
    }
    // other code that might throw IOException
}

In the above example, if BufferedReaderoccurs during the use of IOExceptionor is thrown in subsequent code IOException, BufferedReaderthe object brwill still be properly closed.

That's try-with-resourcesthe advantage of : it ensures that resources declared within its code block are always closed, regardless of exceptions.

2.3.4 The difference between throw and throws

throwand are throwsboth related to exception handling in Java, but their purpose and usage are different. Here are the main differences between them:

1. Uses :

   • throw : is a statement used to explicitly throw an exception in the code. You can use throwto throw custom or existing exceptions.
   • throws : Used to declare exceptions that a method may throw. This doesn't actually throw exceptions, but tells the caller that this method may throw these exceptions, so the caller should either handle them or continue declaring them.

2. location :

   • throw : used in method body.
   • throws : Used on the method signature, immediately after the method's parameter list.

3. Followed by content :

   • throw : followed by an exception object, e.g.throw new Exception("Error message");
   • throws : followed by one or more exception class names, separated by commas, egthrows IOException, SQLException

4. Example :

   use throw:

   public void someMethod() {
         
         
       if (someCondition) {
         
         
           throw new RuntimeException("Some error occurred!");
       }
   }
   

   use throws:

   public void readFile() throws IOException {
         
         
       // ... code that might throw an IOException
   }
   

5. Usage scenario :

   • You can use throwto throw exceptions under certain conditions, or catchto rethrow caught exceptions within a block.
   • You can throwsdeclare exceptions on method signatures to inform callers that they need to be handled or propagated.

6. Processing method :

   • An exception thrown inside throwa method needs to be handled inside the method try-catch, or throwsthe caller can handle it by declaring the exception on the method signature.
   • Exceptions declared with throwscan be caught and handled inside the method, or unhandled and propagated to the method's caller.