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This article mainly explains "what are the Java JVM frequent meeting test questions". The content of 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 are the Java JVM frequent meeting test questions?"

What are the areas of 11.JVM memory and what is the function of each area?

The java virtual machine is mainly divided into the following zones:

Method area:

1. Sometimes it becomes permanent, and garbage collection rarely occurs in this area, but it does not mean that GC does not occur. The GC carried out here is mainly to unload constant pools and types in the method area.

two。 The method area is mainly used to store the information, constants, static variables and code compiled by the real-time compiler of the classes that have been loaded by the virtual machine.

3. This area is shared by threads.

4. There is a run-time pool in the method area that holds literals and symbolic references generated by static compilation. The constant pool is dynamic, that is, constants are not necessarily determined at compile time, and constants generated at run time also exist in this constant pool.

Virtual machine stack:

1. Virtual machine stack, which is commonly known as stack memory, serves java methods. Each method creates a stack frame when it is executed, which is used to store information such as local variables, Operand stacks, dynamic links and method exits.

two。 The virtual machine stack is thread-private and its life cycle is the same as that of threads.

3. Stored in the local variable table are the basic data type, the returnAddress type (the address of a bytecode instruction), and an object reference, which may be a pointer to the starting address of the object, a handle to the object, or the location associated with the object. The memory space required for local variables is determined between compilers

4. The function of the Operand stack is mainly used to store the operation results and operands, which is different from the local variable table accessed through the index, but the way of pressing the stack and leaving the stack.

5. Each stack frame contains a reference to the method to which the stack frame belongs in the runtime constant pool, which is held to support dynamic connections during method calls. Dynamic linking converts symbolic references in constant pools to direct references at run time.

Local method stack

The local method stack is similar to the virtual machine stack, except that the local method stack serves Native methods.

Heap

The java heap is a piece of memory shared by all threads, created when the virtual machine starts, and almost all object instances are created here, so garbage collection operations often occur in this area.

Program counter

Because of the small memory space, the bytecode interpreter can select the next bytecode instruction to be executed by changing this count. The functions such as branching, loop, jump, exception handling and thread recovery all depend on this counter. This memory region is the only one where the java virtual machine specification does not specify any OOM situations.

twelve。 Such as and judge whether an object is alive or not? (or the decision method of GC object)

There are two ways to determine whether an object is alive or not:

1. Citation counting method

The so-called reference counting method is to set a reference counter for each object. whenever there is a place to reference the object, the counter is increased by one, and when the reference expires, the counter is reduced by one. When an object's reference counter is 00:00, the object is not referenced, that is, a "dead object" and will be garbage collected.

A defect of reference counting method is that it can not solve the problem of circular reference, that is to say, when object A refers to object B, and object B refers to object A, then the reference counter of AMagi B object is not zero, which makes it impossible to complete garbage collection, so the mainstream virtual machines do not adopt this algorithm.

two。 Reachability algorithm (reference chain method)

The idea of the algorithm is to search down from an object called GC Roots, and if an object is not linked to GC Roots with any references, the object is not available.

There are several kinds of objects that can be used as GC Roots in java:

Objects referenced in the virtual machine stack

The object referenced by the static property of the method area class

Objects referenced by the method area constant pool

Objects referenced by the local method stack JNI

Although these algorithms can determine whether an object can be recycled, an object ratio may not be recycled when the above conditions are met. When an object is unreachable to GC Root, the object does not

Will not be recycled immediately, but out of a reprieve phase, if you want to be really recycled, you need to go through two marks.

If the object does not have a reference chain with GC Root in the reachability analysis, it is marked for the first time and filtered if it is necessary to execute the finalize () method. When the object does not override the finalize () method or has been called by the virtual machine, it is considered unnecessary.

If it is necessary for the object to execute the finalize () method, the object will be placed in a pair queue called F-Queue, and the virtual machine will trigger a Finalize () thread to execute, which is a low priority thread, and the virtual machine will not promise to wait for it to finish, because if the execution of finalize () is slow or deadlock occurs, it will cause the F-Queue queue to wait. Caused the crash of the memory recovery system. GC marks an object in F-Queue for the second time, at which point the object is removed from the "about to recycle" collection, waiting for recycling.

13. Briefly describe the garbage collection mechanism of java?

In java, programmers do not need to display to free the memory of an object, but are executed by the virtual machine itself. In JVM, there is a garbage collection thread that is low-priority and does not normally execute, triggering execution only when the virtual machine is idle or the current heap is out of memory, scanning objects that are not referenced and adding them to the collection to be recycled.

What are the methods of garbage collection in 14.java?

Mark-clear:

This is the most basic garbage collection algorithm, according to the name can know, its idea is to mark which objects to be recycled, and then unified recycling. This method is simple, but there are two main problems: 1. The efficiency is not high, the efficiency of marking and removal is very low; 2. A large number of discontiguous memory fragments will be generated, causing the program to trigger a GC action in advance because there is not enough continuous memory when the program allocates larger objects.

Replication algorithm:

In order to solve the problem of efficiency, the replication algorithm divides the available memory into two equal parts according to capacity, and then uses only one of them at a time. When one piece of memory is used up, the surviving objects are copied to the second block of memory. Then the first block of memory is clear at one time, and then the objects on the second block are copied to the first block. But in this way, the cost of memory is too high, basically wasting ordinary memory every time.

So the algorithm is improved, the memory area is no longer divided according to 1:1, but the memory is divided into three parts at 8:1:1, the larger part of the memory is transferred to the Eden area, and the remaining two smaller memory areas are called Survior areas. Every time, the Eden area is preferred. If the Eden area is full, the objects are copied to the second memory area, and then the Eden area is cleared. If there are too many objects alive at this time that there is not enough Survivor, these objects will be copied to the old age through the allocation guarantee mechanism. (java reactor is divided into Cenozoic era and old age)

Marking-finishing

The main purpose of this algorithm is to solve the problem of mark-removal, resulting in a large number of memory fragments; when the object survival rate is high, it also solves the efficiency problem of the replication algorithm. The difference is that when the object is cleared, the recyclable object is now moved to one end, and then the object outside the drop boundary is cleared, so that there is no memory fragmentation.

Generational collection

Today's virtual machine garbage collection is mostly used in this way, which divides the heap into the new generation and the old age according to the life cycle of the object. In the new generation, due to the short lifetime of objects, a large number of objects will die each time they are recycled, so the replication algorithm is adopted. In the old days, objects had a high survival rate, and there was no extra space to allocate guarantees, so mark-organize or mark-clear could be used.

15.java memory model

The java memory model (JMM) is the control mechanism for communication between threads. JMM defines the abstract relationship between main memory and threads. Shared variables between threads are stored in main memory (main memory), and each thread has a private local memory (local memory), which stores a copy of the thread to read / write shared variables. Local memory is an abstract concept of JMM and does not really exist. It covers caching, write buffers, registers and other hardware and compiler optimizations. The abstract diagram of the Java memory model is as follows:

From the figure above, if you want to communicate between thread An and thread B, you must go through the following two steps:

1. First, thread A flushes the updated shared variables in local memory A to main memory.

two。 Thread B then goes to main memory to read the shared variables that have been updated by thread A.

16.java class loading process?

Java class loading requires the following seven processes:

Load

The first process of class loading when loading, at this stage, you will accomplish the following three things:

1. Gets the binary stream of a class through the fully qualified name of that class.

two。 The static storage structure in the binary stream is converted into a method to run-time data structure.

3. The Class object of the class is generated in memory as the data access entry for the class.

Verification

The purpose of verification is to ensure that the information in the byte stream of the Class file does not harm the virtual machine. At this stage, the following four verifications are mainly completed:

1. File format verification: verify whether the byte stream conforms to the Class file specification, such as whether the major and minor version numbers are within the scope of the current virtual machine, and whether the constants in the constant pool have unsupported types.

two。 Metadata validation: semantic analysis of the information described by bytecode, such as whether the class has a parent class, whether it integrates classes that are not inherited, etc.

3. Bytecode verification: is the most complex stage in the whole verification process, through the analysis of verification data flow and control flow to determine whether the program semantics is correct, mainly for the verification of the method body. For example, whether the type conversion in the method is correct, whether the jump instruction is correct, and so on.

4. Symbol reference validation: this action occurs later in the parsing process, mainly to ensure that the parsing action is executed correctly.

Prepare for

The preparation phase allocates memory for static variables of the class and initializes it to default values, all of which will be allocated in the method area. The preparation phase does not allocate memory for instance variables in the class, which will be allocated in the Java heap along with the object when the object is instantiated.

The initial value of value for public static int value=123;// is 0 in the preparation phase. It will only become 123 in the initialization phase.

Analysis

This stage mainly completes the conversion action from symbol reference to direct reference. The parsing action is not necessarily before the initialization action is completed, but may also be after initialization.

Initialization

In the last step of class loading during initialization, the previous class loading process is completely dominated and controlled by the virtual machine, except that the user application can participate through the custom class loader in the loading phase. It is not until the initialization phase that the execution of the Java program code defined in the class actually begins.

17. Briefly describe the java class loading mechanism?

The virtual machine loads the data describing the class from the Class file into memory, and verifies, parses and initializes the data, and finally forms the java type that can be directly used by the virtual machine.

18. Parent delegate model mechanism for class loaders?

When a class receives a class load request, it will not load the class first, but delegate it to the parent class, which will be loaded by the parent class. If the parent class cannot be loaded at this time, feedback to the subclass, and the subclass will complete the loading of the class.

19. What are classloaders and what are classloaders?

The block of code that implements the binary byte stream of a class through its permission name is called a class loader.

There are mainly four types of loaders:

1. The startup class loader (Bootstrap ClassLoader) is used to load the java core class library and cannot be directly referenced by java programs.

two。 Extension class loader (extensions class loader): it is used to load the extension library of Java. The implementation of the Java virtual machine provides an extended library directory. The class loader looks for and loads the Java class in this directory.

3. System class loader (system class loader): it loads the Java class based on the classpath (CLASSPATH) of the Java application. Generally speaking, it loads all the classes of Java applications. You can get it through ClassLoader.getSystemClassLoader ().

4. The user-defined class loader is implemented by inheriting the java.lang.ClassLoader class.

20. Brief description of java memory allocation and recovery policy rate, Minor GC and Major GC

Objects are allocated first in the Eden area of the heap.

The big object went straight into the old age.

Those who survive for a long time will go straight into the old age.

When there is not enough space for allocation in the Eden area, a virtual machine execution of Minor GC.Minor Gc usually occurs in the Cenozoic Eden area, where the object survival time is short, the frequency of Gc is high, and the recovery speed is relatively fast. Full Gc/Major GC occurs in the old age. In general, the Minor GC is not triggered when the old GC is triggered, but through configuration, a Minor GC can be performed before the Full GC, which can speed up the recycling speed of the old age.

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