Summary of 35 Java code performance optimizations

Objective

Code optimization, a very important topic. Some people may feel useless, some small places have what good change, change and do not change the efficiency of the operation of the code has any impact? That's the question I'm thinking about, like a whale in the sea, is it useful to eat a little shrimp? No, but after eating a little shrimp, the whale is fed. Code optimization is also the same, if the project focus on as soon as possible without a bug on-line, then you can grasp the large and small, the details of the code can not be fine grinding; but if there is enough time to develop and maintain the code, you have to consider every detail that can be optimized, a small optimization point accumulates, There is definitely an improvement in the efficiency of the code operation.

The goals of code optimization are:

1, reduce the volume of the Code

2, improve the efficiency of code operation

Code optimization Details

1. Specify the final modifier of the class and method as much as possible

A class with a final modifier is not derived. In the Java Core API, there are many examples of final applications, such as Java.lang.String, where the entire class is final. Specifying the final modifier for a class allows the class not to be inherited, and specifying the final modifier for a method allows the method not to be overridden. If a class is specified as final, all methods of the class are final. The Java compiler will look for opportunities to inline all final methods, and inline is important for improving the efficiency of Java operations, see Java Runtime Optimizations. This will increase the performance by an average of 50%.

2. Reuse objects as much as possible

In particular, the use of String objects should be replaced with stringbuilder/stringbuffer when strings are concatenated. Because Java virtual machines take time not only to generate objects, but may also take time to garbage-collect and process them later, generating too many objects will have a significant impact on the performance of the program.

3. Use local variables whenever possible

Parameters passed when the method is called and temporary variables created in the call are saved in the stack faster, and other variables, such as static variables, instance variables, and so on, are created in the heap and are slower. In addition, the variables created in the stack, as the method runs, are gone, and no additional garbage collection is required.

4. Close the flow in time

In the Java programming process, the database connection, I/O flow operation must be careful, when the use is complete, close in time to release resources. Because the operation of these large objects can cause large overhead, and a slight carelessness, will result in serious consequences.

5, minimize the repetition of the variable calculation

To define a concept, the invocation of a method, even if there is only one sentence in the method, is also consumed, including creating a stack frame, protecting the site when invoking a method, recovering the site when the method is called, and so on. So for example, the following actions:

for (int i = 0; i < list.size (); i++) {...}

Suggested substitutions are:

for (int i = 0, int length = List.size (); i < length; i++) {...}

In this way, when the list.size () is very large, it reduces a lot of consumption

6, try to use lazy loading strategy, that is, when needed to create

For example: String str = "AAA"; if (i = = 1) {list.add (str);}

Suggested Replacement: if (i = = 1) {String str = "AAA"; List.add (str);}

7, caution with abnormal abnormal performance unfavorable. Throws an exception first to create a new object, the constructor of the Throwable interface calls the local synchronization method named Fillinstacktrace (), the Fillinstacktrace () method examines the stack, and collects the call trace information. Whenever an exception is thrown, the Java Virtual machine must adjust the call stack because a new object is created during the process. Exceptions can only be used for error handling and should not be used for control procedures.

8, do not use Try...catch in the loop ..., it should be placed in the outermost layer unless the last resort. If there is no reason to write this, as long as your leadership senior, obsessive, probably will scold you why write this garbage code

9, if you can estimate the length of the content to be added, for the underlying array implementation of the collection, the tool class to specify the initial length such as ArrayList, Linkedllist, StringBuilder, StringBuffer, HashMap, HashSet and so on, Take StringBuilder as an example:

(1) StringBuilder ()//default 16 characters allocated space

(2) StringBuilder (int size)//space with size characters assigned by default

(3) StringBuilder (String str)//default allocation 16 characters +str.length () a character space can be set to its initial capacity through a class (which is not just the above StringBuilder), which can significantly improve performance. For example, StringBuilder, length indicates the number of characters the current StringBuilder can hold. Because when the StringBuilder reaches its maximum capacity, it will increase its capacity to the current twice times plus 2, whenever the StringBuilder reaches its maximum capacity, it will have to create a new character array and copy the old character array contents into the new character array-- This is an operation that is very expensive to perform. Imagine, if you can estimate the character array to hold about 5,000 characters without specifying the length, the nearest 5000 of the 2 power is 4096, each expansion plus 2 regardless, then: (1) on the basis of 4096, then apply 8,194 size of the character array, Add up to the equivalent of a 12,290-size character array, if you can initially specify 5,000-size character array, it saves more than one more space (2) to copy the original 4,096 characters into a new character array so that both wasted memory space and reduce the efficiency of code operation. Therefore, it is wrong to set a reasonable initialization capacity for the collection and tool class of the underlying array implementation, which will bring an immediate effect. Note, however, that a collection like HashMap is implemented as an array + linked list, so do not set the initial size to the size you estimate because it is almost 0 more likely to connect an object on a table. The initial size proposal is set to the power of N of 2, which can be set to new HashMap (128), and new HashMap (256) if it can be estimated to have 2000 elements.

10. When copying large amounts of data, use the system.arraycopy () command

11. Multiplication and division use shift operations such as: for (val = 0; Val < 100000; val + = 5) {a = Val * 8; b = VAL/2;} The shift operation can greatly improve performance because at the bottom of the computer, the bitwise operation is the most The fastest, so it is recommended to change to: for (val = 0; Val < 100000; val + = 5) {a = Val << 3; b = Val >> 1;} The shift operation is fast, but may make the code less well understood, Therefore, it is better to add the appropriate comments.

12. Do not constantly create object references within the loop for example: for (int i = 1; I <= count; i++) {Object obj = new Object (); This practice causes the memory to have count copies of the object reference to exist, Coun T is very large, it consumes memory, it is recommended to change to: Object obj = null;for (int i = 0; I <= count; i++) {obj = new object ();} In this case, there is only one copy of the object reference in memory, each Times New Object (), the object reference points to a different object, but only one copy in memory, which saves memory space.

13, based on the efficiency and type checking considerations, should use as far as possible array, cannot determine the size of the array to use ArrayList

14, try to use HashMap, ArrayList, StringBuilder, unless the thread security needs, it is not recommended to use Hashtable, Vector, StringBuffer, the latter three due to the use of synchronization mechanism resulting in performance costs

15. Do not declare an array as public static final because it makes no sense to simply define the reference as static final, the contents of the array can be arbitrarily changed, and declaring the array public is a security vulnerability. This means that the array can be changed by the outer class.

16, as far as possible in the appropriate use of a single case can reduce load burden, shorten the loading time, improve the efficiency of loading, but not all places are applicable to a single case, simple, the single case is mainly applicable to the following three aspects: (1) control the use of resources, through thread synchronization to control the resources of concurrent access (2) Control instances to achieve the purpose of saving resources (3) to control the sharing of data, without establishing a direct association with the conditions, so that multiple unrelated processes or threads to achieve communication between

17, try to avoid arbitrary use of static variables to know, when an object is defined as a variable of static reference, then the GC is usually not recycled the heap memory of this object, such as: public class A {private static b = new B ();} at this time static The life cycle of variable B is the same as Class A, and if Class A is not unloaded, the B object referred to by B will reside in memory until the program terminates

18. Clear a session that is no longer needed in order to clear a session that is no longer active, many application servers have a default session time-out, typically 30 minutes. When the application server needs to save more sessions, if there is not enough memory, the operating system will transfer some of the data to disk, and the application server may dump partially inactive sessions to disk based on the MRU (most recently used) algorithm, and may even throw out-of-memory exceptions. If a session is to be dumped to disk, it must be serialized first, and the cost of serializing the object is expensive in a large-scale cluster. Therefore, when the session is no longer needed, the httpsession invalidate () method should be called in time to clear the session.

19. To implement a collection of randomaccess interfaces such as ArrayList, you should use the most common for loop instead of the Foreach loop to traverse this is what the JDK recommends to the user. The JDK API's explanation for the Randomaccess interface is that the implementation of the Randomaccess interface is used to indicate that it supports fast random access, and the primary purpose of this interface is to allow a generic algorithm to change its behavior so that it can provide good performance when applied to random or contiguous access lists. The actual experience shows that the class instance that implements the Randomaccess interface, if it is random access, uses the normal for loop efficiency higher than the Foreach loop, and conversely, if it is accessed sequentially, the use of iterator is more efficient. You can use code similar to the following to determine: if (list instanceof randomaccess) {for (int i = 0; i < list.size (); i++) {}}else{Iterator Iterator = List.iterable (); while (Iterator.hasnext ()) {Iterator.next ()}} The underlying implementation principle of the Foreach loop is the iterator iterator, see Java Syntax sugar 1: variable length parameters and the Foreach Loop principle. So the latter half of the sentence "in turn, if it is sequential access, the use of iterator will be more efficient" means that sequential access to those class instances, using the Foreach Loop to traverse.

20, using synchronous code block to replace the synchronization method this is in the multi-threaded module synchronized Lock method block in the article has been very clear, unless it can be determined that the entire method is required to synchronize, otherwise try to use synchronous code block, avoid the need to synchronize the code is also synchronized, affect the efficiency of code execution.

21. Declare a constant as static final and name it in uppercase so that it can be put into a constant pool during compilation, avoiding the value of the generated constant during run time. In addition, it is easy to distinguish constants from variables by naming them in uppercase.

22, do not create some non-use of the object, do not import some unused classes this meaningless, if the code appears "the value of the local variable I am not used", "the import java.util is never us Ed ", then please delete these useless content

23. Avoid using reflection during program operation. See Reflection. Reflection is a powerful feature that Java provides to the user, and powerful often means less efficient. It is not recommended to use the Invoke method especially when using the reflection mechanism, especially the method, when the program is running, and if it is necessary, it is suggested that the classes that need to be loaded by reflection will instantiate an object and put it into memory by reflection when the project is started-- The user only cares about the fastest response time when interacting with the peer, and does not care how long it takes to start the project on the end.

24. Use the database connection pool and the thread pool both are used to reuse objects, the former can avoid frequent opening and closing of connections, which avoids the frequent creation and destruction of threads

25, using buffered input and output stream for IO operation with buffered input and output stream, namely BufferedReader, BufferedWriter, Bufferedinputstream, Bufferedoutputstream, This can greatly improve IO efficiency

26, sequential insertion and random access to more scenes using ArrayList, element deletion and intermediate insert more scenes using LinkedList this, understand ArrayList and LinkedList principle will know

27, do not let the public method has too many formal parameter public method that is provided externally, if these methods are too many parameters, there are two disadvantages: 1, violating the object-oriented programming idea, Java emphasizes that everything is object, too many formal parameters, and object-oriented programming ideas do not fit 2, Too many parameters will cause the error probability of the method call to increase as for this "too much" refers to how many, 3, 4 bar. For example, we use JDBC to write a insertstudentinfo method, there are 10 student information fields to be inserted into the student table, you can encapsulate these 10 parameters in an entity class, as the parameter of the Insert method

28, string variable and string constant equals when the string constant is written in front this is a relatively common trick, if there is the following code: string str = "123"; if (Str.equals ("123")) {...} is proposed to be modified to: String str = "123"; if ("123". Equals (str)) {...} This is done primarily to avoid null pointer exceptions

 29, please know that in Java if (i = = 1) and if (1 = = i) is no difference, but from the reading habit, the former is recommended to use, "if (i = = 1)" and "if (1= = i) "There is no difference, this is going to start from the C + +." The "if (i = = 1)" Criterion is determined by 0 and non 0, 0 for false, and not 0 for true if there is such a code: int i = 2; if (i = = 1) {...} Else{...} C + + judgment "I==1″ not set, so 0, that is, false." But if: int i = 2;if (i = 1) {...} else{...} If the programmer is not careful, write "if (i = = 1)" As "if (i = 1)", so there is a problem. If I is assigned to 1,if within if the content is not 0, the return is true, but clearly I is 2, the value of the comparison is 1, should return false. This situation is most likely to occur in the development of C/s + + and can cause some incomprehensible errors, so in order to avoid the developer's incorrect assignment in the IF statement, it is recommended that the IF statement be written as: int i = 2;if (1 = = i) {...} Else{...} This way, even if the developer accidentally writes "1 = i", the C + + compiler can check out the first time because we can assign a value of 1 to a variable, but cannot assign a value of 1 to a constant. However, in Java, the syntax for this "if (i = 1)" is not possible, because once this syntax is written, Java compiles an error "Type mismatch:cannot convert from int to Boolean". However, although Java's "if (i = = 1)" and "if (1 = = i)" Have no semantic differences, it is better to use the former in terms of reading habits.  

30, do not use the ToString () method on an array to see what the array is printed with ToString (): public static void Main (string[] args) {int[] is = new Int[]{1, 2, 3}; System.out.println (Is.tostring ()); The result is: [[email protected] originally intended to print an array of contents, but it is possible that a null pointer exception is caused because the arrays reference is null. However, although the array ToString () does not make sense, the set ToString () is able to print out the contents of the collection, because the parent of the collection Abstractcollections overrides the ToString () method of object.  

31. Do not make a downward transition to the underlying data type that is out of range this will never get the result you want: public static void Main (string[] args) {long L = 12345678901234L; int i = (int) l; S Ystem.out.println (i); We may expect to get some of them, but the result is: 1942892530 to explain. In Java, Long is 8 bytes 64 bits, so 12345678901234 in the computer representation should be: 0000 0000 0000 0000 0000 1011 0011 1010 0111 0011 1100 1110 0010 1111 1111 101 int data is 4 bytes and 32 bits, the first 32 bits of the binary data are removed from the lower level: 0111 0011 1100 1110 0010 1111 1111 0010 This string binary is represented as a decimal 1942892530, so it's the console above us. The content of the output. In this case, you can also get two conclusions: 1, the default data type of the integer is Int,long l = 12345678901234L, this number is beyond the range of int, so there is an L, which indicates that this is a long type. By the way, the default type of float is double, so define float as "float f = 3.5f" 2, then write "int II = L + i;" Error, because long + int is a long and cannot be assigned to int

32, the Common collection class does not use the data must be removed in time if a collection class is common (that is, it is not a property within the method), then the elements inside the collection are not automatically freed, because references always point to them. Therefore, if some of the data in a common collection is not used without going to remove them, it will cause this common set to grow, causing the system to have a hidden memory leak.

33. To convert a basic data type to a string, the basic data type. ToString () is the quickest way, string.valueof (data), Data + "" slowest to convert a basic data type to a general three ways, I have an integer data i, You can use i.ToString (), string.valueof (i), i+ "" Three ways, three ways of efficiency, see a test: public static void Main (string[] args) {int looptime = 50000; Integer i = 0; Long startTime = System.currenttimemillis (); for (int j = 0; J < Looptime; J + +) {String str = string.valueof (i);} System.out.println ("string.valueof ():" + (System.currenttimemillis ()-StartTime) + "MS"); StartTime = System.currenttimemillis (); for (int j = 0; J < Looptime; J + +) {String str = i.tostring ();} System.out.println ("integer.tostring ():" + (System.currenttimemillis ()-StartTime) + "MS"); StartTime = System.currenttimemillis (); for (int j = 0; J < Looptime; J + +) {String str = i + "";} System.out.println ("i + \" \ ":" + (System.currenttimemillis ()-StartTime) + "MS"); The result of the operation is: String.valueof (): 11ms integer.tostring (): 5ms i + "": 25ms Therefore, the toString () method is preferred when you encounter the conversion of a basic data type to String. As for why, it is very simple: 1, string.valueof () method Bottom Call integer.tOstring () method, but will be short before the call Judgment 2, integer.tostring () method does not say, directly called 3, i + "" bottom use of StringBuilder implementation, first with Append method splicing, and then with ToString () method to get a string of three comparisons, is clearly 2 the fastest, 1 times, 3 slowest  

34, the most efficient way to traverse the map to traverse the map has a lot of ways, usually we need to traverse the map key and value, then the most efficient way to recommend the use of: public static void Main (string[] args) { HashMap HM = new HashMap (); Hm.put ("111", "222"); set> EntrySet = Hm.entryset (); Iterator> iter = Entryset.iterator (); while (Iter.hasnext ()) {Map.entry Entry = Iter.next (); System.out.println (Entry.getkey () + "\ T" + entry.getvalue ()); }} If you just want to traverse the key value of this map, use "Set KeySet = Hm.keyset ();" would be more appropriate.

35, to the resources of Close () suggested separate operation means, for example I have such a piece of code: try{xxx.close (); Yyy.close (); }catch (Exception e) {...} proposed modification to: try{xxx.close ();} catch (Exception e) {...} try{yyy.close ();} catch (Exception e) {...} is a bit of a hassle but can avoid resource leaks. We think, if there is no modified code, in case Xxx.close () throws an exception, then into the Cath block, Yyy.close () will not execute, YYY this resource will not be recycled, has been occupied, such a lot of code, is likely to cause the disclosure of resource handle. And instead of the following wording, it is guaranteed that XXX and yyy will be close off anyway.

Summary of 35 Java code performance optimizations