Meet java-17.4 in the beginning (5)-Deadlock

In this chapter we will briefly discuss the deadlock, in fact, the deadlock is a relatively large topic, but I am here just according to the example of bank transfer in front of me, there is a detailed column to illustrate the various aspects of concurrent programming problems.

1. What is a deadlock?

Refers to two or more than two processes in the course of execution, because of competing resources or due to the communication between each other caused by a blocking phenomenon, if there is no external force, they will not be able to proceed.

2. How does it cause a deadlock?

(1) Mutually exclusive condition: A resource can only be used by one process at a time;

(2) Request and maintain conditions: holding the existing resources, but also to apply for other resources, but the application is not always blocked;

(3) Non-deprivation: the resources for the use of resources cannot be deprived by priority;

(4) Cyclic waiting condition: a cyclic waiting resource relationship is formed between several processes.

3. How did the previous example cause a deadlock?

(1) The process of deadlock generation:

Account 1 Original $200
Account 2 Original $300
Thread 1: From account 1 to 300 yuan to account 2
Thread 2: From account 2 to 400 yuan to account 1

(2) A block of code that causes a deadlock:

public void Transfer (int fromaccount, int. Toaccount, double money) {if (Accounts[fromaccount] < money) {System.out.prin TLN (Thread.CurrentThread (). GetName () + "deadlock appears"); return;} Accounts[fromaccount]-= money; System.out.printf ("from" + Fromaccount + "account transfer%10.2f Yuan,", money); Accounts[toaccount] + = money; System.out.printf ("from" + Toaccount + "account transferred to%10.2f Yuan,", money); System.out.printf ("Total:%10.2f yuan", gettotal ()); System.out.println ();}

(3) Reason:

When the money transferred is larger than the existing balance, a deadlock will be generated

(4) Solution:

However, because the real business does not allow this situation to occur, so we directly return to solve the deadlock problem.

(5) If it is other multi-thread competing resources, it is not the same as the above return, and wait, there will be a deadlock situation.

4. Complete Example:

Package Com.ray.ch17;public class Bank {private final double[] Accounts;public double[] getaccounts () {return accounts;}  public Bank (int n, double initbalance) {accounts = new double[n];for (int i = 0; i < accounts.length; i++) {accounts[i] = Initbalance;}} Public double gettotal () {Double total = 0;for (int i = 0; i < accounts.length; i++) {total + = accounts[i];} return total;} public void Transfer (int fromaccount, int. Toaccount, double money) {if (Accounts[fromaccount] < money) {System.out.prin TLN (Thread.CurrentThread (). GetName () + "deadlock"),//Thread.CurrentThread (). Wait ();//If it is another multi-threaded, it may be wait here, Thus a true deadlock return appears; Accounts[fromaccount]-= money; System.out.printf ("from" + Fromaccount + "account transfer%10.2f Yuan,", money); Accounts[toaccount] + = money; System.out.printf ("from" + Toaccount + "account transferred to%10.2f Yuan,", money); System.out.printf ("Total:%10.2f yuan", gettotal ()); System.out.println ();} public int size () {return accounts.length;}}

Package Com.ray.ch17;import Java.util.random;public class Transferthread implements Runnable {private Bank bank;private Final double max;public Transferthread (Bank bank, double MAX) {This.bank = Bank;this. max = max;} @Overridepublic void Run () {while (true) {Double amount = MAX * math.random (); int countofaccount = Bank.getaccounts (). Leng Th;bank.transfer (New Random (). Nextint (Countofaccount), New Random (). Nextint (Countofaccount), amount);}}

Package Com.ray.ch17;public class Test {public static void main (string[] args) {Bank Bank = new Bank (+ 10000); for (int i = 0; I < 2; i++) {Transferthread transferthread = new Transferthread (bank, 10000); Thread thread = new Thread (transferthread); Thread.Start ();}}}

Summary: This section provides a brief introduction to deadlocks.

This chapter is here, thank you.

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Meet java-17.4 in the beginning (5)-Deadlock