Summary of 35 Java code performance optimizations

The goals of code optimization are:

1, reduce the volume of the Code

2, improve the efficiency of code operation

Code optimization details 1, specifying 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 (intint 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 substitutions are:

if (i = =1 = "AAA"; List.add (str);}

7, cautious use of abnormal

Exceptions are bad for performance. 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 ..., you should put it on the outermost

Unless there is a 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, specify the initial length for the collection that the underlying array implements, the tool class

such as ArrayList, Linkedllist, StringBuilder, StringBuffer, HashMap, HashSet, and so on, take StringBuilder as an example:

(1) StringBuilder ()//16-character space is assigned by default

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

(3) StringBuilder (String str)//default 16 characters +str.length () character space

It is possible to set its initialization capacity through the class (which refers to not just the StringBuilder above), 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, and 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 a 5,000-size character array, saving more than a space

(2) Copy the original 4,096 characters into the new character array.

In this way, both the memory space is wasted and the code runs efficiently. 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. Use shift operations with system.arraycopy () command 11, multiplication, and division when copying large amounts of data

For example:

 for (val = 0; Val < 100000; val + =5 = val *8 = VAL/2;}

The shift operation can greatly improve performance, because at the bottom of the computer, the bitwise operation is the most convenient and fastest, so it is recommended to modify:

 for (val = 0; Val < 100000; val + =5 = val <<3 = Val >> 1;}

Although the shift operation is fast, it may make the code less understandable, so it's best to add the appropriate comments.

12. Do not constantly create object references within the loop

For example:

 for (int i = 1; I <= count; i++new  Object ();}

This practice causes memory to count as the object reference exists, and if count is large, it consumes memory and is recommended instead:

null;  for (intnew Object ();}

In this case, only one copy of the object object is referenced in memory, and each time new object () is used, the object reference points to a different object, but only one copy in memory, which saves memory space considerably.

13, based on the efficiency and type checking considerations, should use the array as far as possible, can not determine the size of arrays to use ArrayList14, try to use HashMap, ArrayList, StringBuilder, unless the thread security needs, Otherwise, it is not recommended to use Hashtable, Vector, StringBuffer, the latter three due to the use of synchronization mechanism caused by the performance cost 15, do not declare the array as public static final

Because this is meaningless, it simply defines the reference as static final, the contents of the array can be arbitrarily changed, and declaring the array public is a security vulnerability, which means that the array can be changed by the outer class

16, try to use a single case in the appropriate situation

Using a single example can reduce the load burden, shorten the load time, improve the efficiency of loading, but not all places are applicable to a single case, simply speaking, the single case is mainly applicable to the following three aspects:

(1) Control the use of resources, through thread synchronization to control the concurrent access of resources

(2) control of the production of instances to achieve the purpose of saving resources

(3) Controlling the sharing of data, allowing multiple unrelated processes or threads to communicate without establishing a direct association

17, try to avoid arbitrary use of static variables

You know, when an object is referenced by a variable defined as static, the GC usually does not reclaim the heap memory that the object occupies, such as:

 Public class  Privatestaticnew  B ();}

At this point, the life cycle of static 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. Timely removal of sessions that are no longer needed

In order to clear a session that is no longer active, many application servers have a default session timeout, which typically takes 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, the implementation of the Randomaccess interface, such as ArrayList, 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 like this to make a decision:

if instanceof   for (int i = 0; i < list.size (); i++) {}}else{Iterator  while (Iterator.hasnext ()) {Iterator.next ()}}

The underlying implementation 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. Use synchronous code block instead of synchronization method

This is clearly stated in the synchronized lock method block in multithreaded modules, unless you can determine that a whole method needs to be synchronized, try to use synchronous blocks of code, avoid synchronizing code that does not need to be synchronized, and affect the efficiency of code execution.

21. Declare a constant as static final and name it in uppercase

This allows the content to be placed in 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 unused objects, do not import some unused classes

This does not make sense if the code appears "the value of the local variable I am not used", "the import java.util is never used", then remove these useless content

23. Avoid using reflection during program operation

about, 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. Using the database connection pool and the thread pool

Both pools are used to reuse objects that avoid frequent opening and closing of connections, which avoids the frequent creation and destruction of threads

25. IO operation with buffered input/output stream

Buffered input and output streams, BufferedReader, BufferedWriter, Bufferedinputstream, Bufferedoutputstream, which can greatly improve IO efficiency

26, sequential insertion and random access more scenes using ArrayList, element deletion and intermediate insertions more scenes using LinkedList

This, understand the principles of ArrayList and LinkedList.

27. Do not let the public method have too many formal parameters

The public method is a method provided externally, and there are two disadvantages to giving these methods too many parameters:

1, violates the object-oriented programming idea, Java stresses that everything is object, too many formal parameters, and object-oriented programming thinking does not fit

2, too many parameters will inevitably lead to the error probability of the method call increased

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 little trick, if you have the following code:

String str = "123"; if (Str.equals ("123")) {...}

Suggested changes to:

String str = "123"; if ("123". Equals (str)) {...}

This is done primarily to avoid null pointer exceptions

29, please know, in Java if (i = = 1) and if (1 = = i) is no difference, but from the reading habit, the use of the former is recommended

At ordinary times someone asked, "if (i = = 1)" and "if (1== i)" There is no difference, this will start from 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 piece of 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 { ... }

In case the programmer is careless, 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 { ... }

Summary of 35 Java code performance optimizations