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We know that multithreading can process multiple tasks concurrently, effectively improve the performance of complex applications, and plays a very important role in practical development
However, using multithreading also brings many risks, and the problems caused by threads are often difficult to find in the test, and will cause major failures and losses on the line
I will combine several practical cases to help you avoid these problems in your work
Multithreading problem
First of all, let’s look at some of the problems with using multiple threads
The problem with multithreading is largely due to the multiple threads’ right to operate on the same variable and the uncertainty of the execution order between different threads
The book “Java Concurrent Programming in Action” addresses three kinds of multithreading problems: security problems, activity problems, and performance problems
Security issues
For example, there is a very simple storage function operation, as follows:
public int decrement(a){
return --count;//count initial inventory is 10
}
Copy the codeIn a single-threaded environment, this method works correctly, but in a multi-threaded environment, it leads to incorrect results
–count looks like an operation, but it actually contains three steps (read, modify, write) :
- Read the value of count
- The value minus one
- Finally, we assign the result to count
The following figure shows an incorrect execution. When two threads (1 and 2) execute the method simultaneously, they both read count as 10 and return a result of 9. This means that two people may have purchased the product, but the inventory is only reduced by 1, which is unacceptable for a real production environment
A situation like the one above that results in incorrect results due to improper execution timing is a very common concurrency safety problem known as a race condition
The decrement() method, the section of code that causes race conditions to occur, is called a critical section
To avoid this problem, you need to ensure the atomicity of the read-modify-write compound operation
In Java, there are many ways to implement this, including the Synchronize built-in lock or ReentrantLock explicit lock mechanism, the use of thread-safe atomic classes, and the CAS approach
Activity problem
An activity problem is when an operation cannot continue because it is blocked or looping
The three most typical are deadlock, live lock and starvation
A deadlock
The most common activity problem is deadlocks
Deadlocks refer to multiple threads waiting for each other to obtain the lock, but will not release their own lock, and cause the blocking of these threads can not run is deadlocks, it is often caused by the incorrect use of locking mechanism and the unpredictability of execution order between threads
How can deadlocks be prevented
1. Ensure that the lock sequence is the same
For example, there are locks A,B and C.
- The lock sequence of Thread 1 is A, B, and C.
- Thread 2 is locked in sequence A and C to avoid deadlocks.
If Thread2 is locked in an inconsistent order B/A, or C/A, deadlock will occur.
2. Use the time-out waiver mechanism whenever possible
The Lock interface provides the tryLock(long time, TimeUnit Unit) method, which can wait for a Lock for a fixed period of time, so that the thread can voluntarily release all the locks it has acquired after the Lock expires. Deadlock issues can be avoided
Live lock
A live lock is very similar to a deadlock in that the program never waits for a result, but in contrast to a deadlock, a live lock is alive. Because the running thread is not blocking, it is always running, but it never gets the result
hunger
Hunger is the problem of a thread not being able to run when it needs certain resources, especially CPU resources.
In Java, there is the concept of thread priority, with Java priorities ranging from 1 to 10, with 1 lowest and 10 highest.
If we set the priority of a thread to 1, which is the lowest priority, in this case, the thread may never be allocated CPU resources, causing it to fail for a long time.
Performance issues
Thread creation and switching between threads consume resources. If frequent thread creation or CPU scheduling takes longer than the running time of the thread, the use of thread is not worth the loss, and may even result in high CPU load or OOM consequences
For example
Thread-unsafe classes
Case 1
For synchronization using thread-unsafe collections (ArrayList, HashMap, and so on), it is best to use thread-safe concurrent collections
In a multithreaded environment, collection traversal of thread unsafe operation, may throw ConcurrentModificationException abnormalities, is often said that fail – fast mechanism
The following example simulates multiple threads working on an ArrayList at the same time, thread T1 iterating through the list and printing, and thread T2 adding elements to the list
List<Integer> list = new ArrayList<>();
list.add(0);
list.add(1);
list.add(2); / / the list: [0]
System.out.println(list);
// Thread T1 iterates through the printed list
Thread t1 = new Thread(() -> {
for(inti : list){ System.out.println(i); }});// Thread T2 adds elements to list
Thread t2 = new Thread(() -> {
for(int i = 3; i < 6; i++){ list.add(i); }}); t1.start(); t2.start();Copy the code
Looking into the source code for the exception throwing ArrayList, you can see that traversing the ArrayList is done through an iterator that is implemented internally
When the iterator’s next() method is called to get the next element, the checkForComodification() method is used to check whether modCount and expectedModCount are equal. If he is not equal to throw ConcurrentModificationException
ModCount is a property of ArrayList that represents the number of times the collection structure has been modified (the number of times the list length has changed), and modCount is incremented by one each time a method like add or remove is called
ExpectedModCount is an iterator property that is assigned the same value as the prior modCount when the iterator instance is created (expectedModCount=modCount)
So when other threads add or remove set elements, modCount increases, and then the set traversal expectedModCount is not equal to modCount, and an exception is thrown
Use locking to manipulate thread-unsafe collection classes
List<Integer> list = new ArrayList<>();
list.add(0);
list.add(1);
list.add(2);
System.out.println(list);
// Thread T1 iterates through the printed list
Thread t1 = new Thread(() -> {
synchronized (list){ // Use the synchronized keyword
for(inti : list){ System.out.println(i); }}});// Thread T2 adds elements to list
Thread t2 = new Thread(() -> {
synchronized (list){
for(int i = 3; i < 6; i++){ list.add(i); System.out.println(list); }}}); t1.start(); t2.start();Copy the codeAs in the code above, locking operations on the list with the synchronized keyword does not throw an exception. Synchronized, however, serializes locked blocks of code and is not advantageous in terms of performance
Thread-safe concurrency utility classes are recommended
JDK1.5 adds many thread-safe utility classes for use with concurrent containers like CopyOnWriteArrayList and ConcurrentHashMap
These utility classes are recommended for daily development for multithreaded programming
Case 2
Do not use SimpleDateFormat as a global variable
SimpleDateFormat is actually a thread-unsafe class, and the root cause is that SimpleDateFormat’s internal implementation does not synchronize operations on some shared variables
public static final SimpleDateFormat SDF_FORMAT = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
public static void main(String[] args) {
// Both threads call the simpleDateformat.parse method simultaneously
Thread t1 = new Thread(() -> {
try {
Date date1 = SDF_FORMAT.parse("The 2019-12-09 17:04:32");
} catch(ParseException e) { e.printStackTrace(); }}); Thread t2 =new Thread(() -> {
try {
Date date2 = SDF_FORMAT.parse("The 2019-12-09 17:43:32");
} catch(ParseException e) { e.printStackTrace(); }}); t1.start(); t2.start(); }Copy the code
It is recommended to use SimpleDateFormat as a local variable or in conjunction with ThreadLocal
The simplest way is to use SimpleDateFormat as a local variable
However, if used in a for loop, many instances are created and can be optimized to work with ThreadLocal
/ / initialization
public static final ThreadLocal<SimpleDateFormat> SDF_FORMAT = new ThreadLocal<SimpleDateFormat>(){
@Override
protected SimpleDateFormat initialValue(a) {
return new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); }};/ / call
Date date = SDF_FORMAT.get().parse(wedDate);
Copy the codeJava8 LocalDateTime and DateTimeFormatter are recommended
LocalDateTime and DateTimeFormatter are new features introduced in Java 8 that are not only thread-safe, but also easier to use
It is recommended to replace Calendar and SimpleDateFormat with LocalDateTime and DateTimeFormatter in practical development
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
LocalDateTime time = LocalDateTime.now();
System.out.println(formatter.format(time));
Copy the codeCorrect lock release
Suppose you have pseudocode like this:
Lock lock = newReentrantLock(); .try{
lock.tryLock(timeout, TimeUnit.MILLISECONDS)
// Business logic
}
catch (Exception e){
// Error log
// Throw an exception or return directly
}
finally {
// Business logiclock.unlock(); }...Copy the codeIn this code, a piece of business logic is executed before the finally block releases the lock
If the dependent service in this logic is not available, the thread that occupies the lock cannot release the lock successfully, it will cause other threads to block because they cannot acquire the lock, and eventually the thread pool will be full
So before you release the lock; In the finally clause, there should be only some handling of releasing resources (such as locks, IO streams, etc.) held by the current thread
Another is to set a reasonable timeout when acquiring the lock
To prevent a thread from blocking without a lock, a timeout period can be set. When the lock expires, the thread can throw an exception or return an error status code. The timeout time should be reasonable, not too long, and should be longer than the execution time of the locked business logic.
Use thread pools correctly
Case 1
Do not use thread pools as local variables
public void request(List<Id> ids) {
for (int i = 0; i < ids.size(); i++) { ExecutorService threadPool = Executors.newSingleThreadExecutor(); }}Copy the codeCreating a thread pool in the for loop creates as many thread pools as the size of the list of input arguments each time the method executes, and the shutdown() method is not called in time to destroy the thread pool
In this case, as more requests come in, the thread pool will consume more and more memory, leading to frequent fullGC and even OOM. It makes no sense to create a thread pool for each method call, as it is no different from creating and destroying threads yourself frequently, not only without taking advantage of the thread pool, but also consuming more resources required by the thread pool
So try to use thread pools as global variables
Case 2
Use the default thread pool static method with caution
Executors.newFixedThreadPool(int); // Create a thread pool with a fixed capacity
Executors.newSingleThreadExecutor(); // Create a thread pool of capacity 1
Executors.newCachedThreadPool(); // Create a thread pool with the capacity of integer.max_value
Copy the codeRisk points for the three default thread pools above:
NewFixedThreadPool creates thread pools corePoolSize and maximumPoolSize with equal values and uses a blocking queue called LinkedBlockingQueue.
NewSingleThreadExecutor sets both corePoolSize and maximumPoolSize to 1 and also uses LinkedBlockingQueue
LinkedBlockingQueue has a default size of INTEger. MAX_VALUE=2147483647, which is considered an unbounded queue for a real machine
- If the number of threads running newFixedThreadPool and newSingleThreadExecutor exceeds the number of threads running corePoolSize, subsequent requests will be placed in a blocking queue. Because the blocking queue is too large, subsequent requests will not fail quickly and will be blocked for a long time. The thread pool on the request side can be overwhelmed, dragging down the entire service.
NewCachedThreadPool sets corePoolSize to 0, maximumPoolSize to integer. MAX_VALUE, blocks the SynchronousQueue used by the queue, SynchronousQueue does not hold tasks waiting to execute
- So newCachedThreadPool creates threads to run as tasks come in, and maximumPoolSize equals an unlimited setting, so that the number of threads created may fill up the machine’s memory.
Therefore, you need to create custom thread pools based on your services and hardware configurations
Number of threads
Set the number of thread pool corePoolSize.
1. Cpu-intensive applications
Cpu-intensive means that tasks require a lot of complex calculations, little blocking, and require the CPU to run at high speed for long periods of time.
General formula: corePoolSize= number of CPU cores +1 thread The number of CPU cores that the JVM can run can be viewed at Runtime.getruntime ().availableProcessors().
2.IO intensive applications
IO intensive tasks involve a lot of disk reads and writes or network transfers, and threads spend more time on I/O blocking than CPU processing. Most business applications are IO intensive.
Reference formula: Optimal number of threads = number of cpus /(1- blocking factor); Blocking factor = thread wait time /(thread wait time +CPU processing time).
The CPU processing time of IO intensive tasks is often much shorter than the thread waiting time, so the blocking coefficient is generally considered to be between 0.8-0.9. For a 4-core single-slot CPU, corePoolSize can be set to 4/(1-0.9)=40. Of course, the specific Settings should be based on the actual operation of the machine
The last
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Reference Books:
- Java concurrent programming practice