Difference between heap and stack in Java

Both stacks and stacks are places where Java is used to store data in RAM. Unlike C ++, Java automatically manages stacks and stacks, and programmers cannot directly set stacks or stacks. The Java heap is a runtime data zone, from which the class objects allocate space. These objects are created using commands such as new, newarray, anewarray, and multianewarray. They do not need program code to be explicitly released. The heap is responsible for garbage collection. The advantage of the heap is that the memory size can be dynamically allocated, and the lifetime does not have to be told in advance because the heap dynamically allocates memory at runtime, the Java Garbage Collector automatically collects the unused data. However, the slow access speed is due to the need to dynamically allocate memory during runtime. The advantage of stack is that the access speed is faster than that of stack, second only to register, and stack data can be shared. However, the disadvantage is that the data size and lifetime in the stack must be fixed, and there is a lack of flexibility. The stack mainly stores some basic types of variables (, int, short, long, byte, float, double, boolean, char) and object handles. A very important feature of stacks is that data in stacks can be shared. Suppose we define both: 　　int a = 3;　　int b = 3; The compiler first processes int a = 3. First, it creates a reference with the variable a in the stack, and then finds whether the value 3 in the stack exists. If no value is found, store 3 and point a to 3. Then process int B = 3. After the referenced variable of B is created, B is directed to 3 because there is already 3 in the stack. In this way, both a and B point to 3 at the same time. At this time, if a is set to 4 again, the compiler will re-search whether there are 4 values in the stack. If not, it will store 4 and make a point to 4; if yes, direct a to this address. Therefore, changing the value of a does not affect the value of B. Note that the sharing of data is different from the sharing of two objects pointing to one object at the same time, because the modification of a does not affect B, which is completed by the compiler, it facilitates space saving. A variable referenced by an object modifies the internal state of the object, which affects the variable referenced by another object. 　　StringIs a special packaging data. Available: 　　String str = new String("abc");　　String str = "abc"; The first method is to use new () to create an object, which is stored in the heap. Each call creates a new object. The second is to first create a String class object in the stack to reference the variable str, and then find whether the stack contains "abc". If not, store "abc" into the stack and point str to "abc". If "abc" already exists, direct str to "abc ". Use the equals () method to compare the values in a class. Use the = method to test whether the references of the two classes point to the same object. The example below illustrates the above theory. 　　String　str1　=　"abc";　　String　str2　=　"abc";　　System.out.println(str1==str2);　//true   It can be seen that str1 and str2 point to the same object. 　　String　str1　=new　String　("abc");　　String　str2　=new　String　("abc");　　System.out.println(str1==str2);　//　false   The new method is used to generate different objects. Each time one is generated. Therefore, the first method is used to create multiple "abc" strings. Only one object exists in the memory. this method is advantageous and saves memory space. at the same time, it can improve the program running speed to a certain extent, because the JVM will automatically decide whether to create a new object based on the actual situation of the data in the stack. For the code of String str = new String ("abc");, a new object is created in the heap, regardless of whether the String value is equal, whether it is necessary to create a new object, this increases the burden on the program. On the other hand, NOTE: When we define classes using formats such as String str = "abc";, we always take it for granted that the str object of the String class is created. Worry trap! The object may not be created! Instead, it may only point to a previously created object. Only by using the new () method can a new object be created every time. Because of the immutable property of the String class, when the String variable needs to change its value frequently, you should consider using the StringBuffer class to improve program efficiency. 2.2 system response after application STACK: as long as the remaining space of the stack exceeds the applied space, the system will provide the program with memory. Otherwise, an exception will be reported, prompting stack overflow. Heap: First, you should know that the operating system has a linked list that records idle memory addresses. When the system receives a program application, it will traverse the linked list, find the heap node with the first space greater than the requested space, delete the node from the idle node linked list, and allocate the space of the node to the program. In addition, for most systems, the size of the allocation will be recorded at the first address in the memory space, so that the delete statement in the code can correctly release the memory space. In addition, because the size of the heap node is not necessarily equal to the applied size, the system automatically places the excess part in the idle linked list. 2.3 Application size limit STACK: in Windows, a stack is a data structure extended to a low address and a continuous memory area. This statement indicates that the stack top address and the maximum stack capacity are pre-defined by the system. In WINDOWS, the stack size is 2 MB (OR 1 MB, it is a constant determined during compilation. If the requested space exceeds the remaining space of the stack, overflow is displayed. Therefore, the space available from the stack is small. [Page] Heap: the heap is a data structure extended to the high address and a non-sequential memory area. This is because the system uses the linked list to store the idle memory address, which is naturally discontinuous, And the traversal direction of the linked list is from the low address to the high address. The heap size is limited by the valid virtual memory in the computer system. It can be seen that the space obtained by the heap is flexible and large. 2.4 comparison of application efficiency: The stack is automatically allocated by the system, which is faster. But programmers cannot control it. The heap is the memory allocated by new, which is generally slow and prone to memory fragments. However, it is most convenient to use. In addition, in WINDOWS, the best way is to use VirtualAlloc to allocate memory. Instead of heap or stack, it is to reserve a fast memory in the address space of the process, although it is the most inconvenient to use. However, it is fast and flexible. Storage content in 2.5 heap and stack STACK: when calling a function, the first entry to the stack is the address of the next instruction in the main function (the next executable statement in the function call statement), and then the parameters of the function, in most C compilers, parameters are written from right to left into the stack, followed by local variables in the function. Note that static variables are not included in the stack. When the function call ends, the local variable first goes out of the stack, then the parameter, and the top pointer of the stack points to the address of the initial storage, that is, the next instruction in the main function, where the program continues to run. Heap: Generally, the heap size is stored in one byte in the heap header. The specific content in the heap is arranged by the programmer. 2.6 comparison of access efficiency 　　char s1[] = "aaaaaaaaaaaaaaa";　　char *s2 = "bbbbbbbbbbbbbbbbb"; Aaaaaaaaaaa is assigned a value at the runtime; Bbbbbbbbbbbbb is determined during compilation; However, in future access, the array on the stack is faster than the string pointed to by the pointer (such as the heap. For example: 　　void　main()　　{　　char　a　=　1;　　char　c[]　=　"1234567890";　　char　*p　="1234567890";　　a　=　c[1];　　a　=　p[1];　　return;　　}   Corresponding assembly code 　　10:　a　=　c[1];　　00401067　8A　4D　F1　mov　cl,byte　ptr　[ebp-0Fh]　　0040106A　88　4D　FC　mov　byte　ptr　[ebp-4],cl　　11:　a　=　p[1];　　0040106D　8B　55　EC　mov　edx,dword　ptr　[ebp-14h]　　00401070　8A　42　01　mov　al,byte　ptr　[edx+1]　　00401073　88　45　FC　mov　byte　ptr　[ebp-4],al   The first method reads the elements in the string directly to the cl register, while the second method reads the pointer value to the edx. Reading characters by edx is obviously slow. 2.7 summary: The difference between stack and stack can be seen in the following metaphor: Using Stacks is like eating at a restaurant, just ordering food (sending an application), paying for it, and eating (using it). If you are full, you can leave, without having to worry about the preparation work, such as cutting and washing dishes, as well as the cleaning and tail work such as washing dishes and flushing pots, his advantage is fast, but his degree of freedom is small. Using heap is like making your favorite dishes. It is troublesome, but it suits your taste and has a high degree of freedom.