Java Learning Note 46 (multithreading three: communication between threads)

Multiple threads are working on the same resource, but the tasks of the threads are not the same, so that each thread can use the resources efficiently by some means.

This means: Waiting for a wake-up mechanism, also known as communication between threads

Methods involved: Wait (), notify ()

Example:

Two threads one input, one output

 Package demo;  Public class Resource {    public  String name;      Public String sex;}

Input thread:

 Packagedemo; Public classInputImplementsRunnable {PrivateResource r =NewResource ();  Public voidrun () {inti = 0;  while(true) {            if(i% 2 = = 0) {R.name= "Zhang San"; R.sex= "Male"; } Else{r.name= "John Doe"; R.sex= "female"; } I++; }    }}

Output thread:

 Package demo;  Public class Implements Runnable {    privatenew  Resource ();      Public void run () {        while (true) {            System.out.println (r.name+ "..." + r.sex);     }}}

Test class:

 Package demo;  Public class Threaddemo {    publicstaticvoid  main (string[] args) {        new  Input ();         New Output ();         New Thread (in);         New Thread (out);                Tin.start ();        Tout.start ();    }}

After running, we find that the output is Null...null

Because the input thread and the resource object created in the output thread make different

Resolve NULL Issues:

 Packagedemo; Public classInputImplementsRunnable {PrivateResource R;  PublicInput (Resource r) { This. R =R; }     Public voidrun () {inti = 0;  while(true) {            if(i% 2 = = 0) {R.name= "Zhang San"; R.sex= "Male"; } Else{r.name= "John Doe"; R.sex= "female"; } I++; }    }}

 package   demo;  public  class  Output implements   Runnable { private   Resource R;  public   Output (Resource r) { this . R = R;  public  void   run () { while  (true   + "..." +r.sex); }    }}

 Packagedemo; Public classThreaddemo { Public Static voidMain (string[] args) {Resource R=NewResource (); Input in=NewInput (R); Output out=NewOutput (R); Thread Tin=NewThread (in); Thread Tout=NewThread (out);        Tin.start ();    Tout.start (); }}

Another problem was discovered after running:

The output contains: Zhang San ... Female or Li four ... Male, sex error

Cause of Occurrence:

After the assignment of Zhang San and male, continue to assign value John Doe and female, this time has not yet and the assignment of female, entered the output thread, then output John Doe ... Man

So I thought of adding synchronization:

     Public voidrun () {inti = 0;  while(true) {            synchronized( This) {                if(i% 2 = = 0) {R.name= "Zhang San"; R.sex= "Male"; } Else{r.name= "John Doe"; R.sex= "female"; } I++; }        }    }

     Public void run () {        while (true) {            synchronized (this ) {                 + "..." + r.sex);     }}}

However, the problem has not been solved:

Reason:

The synchronization here has lost its function, not a lock.

Workaround:

Use a common lock to

 Public voidrun () {inti = 0;  while(true) {            synchronized(r) {if(i% 2 = = 0) {R.name= "Zhang San"; R.sex= "Male"; } Else{r.name= "John Doe"; R.sex= "female"; } I++; }        }    }

     Public void run () {        while (true) {             synchronized  (r)                { + "..." + r.sex);     }}}

This is the normal output.

But there is still a problem, we hope that the Zhang San and John Doe staggered, a 31 John Doe, is still random, large tracts of Zhang San or John Doe

Workaround:

Let the input thread assign a value, then let the output thread out, and let the incoming thread wait, not allow the assignment of John Doe, wait for output Zhang San end, then allow Li Shiyi value, go down

The input thread also needs the same way, and waits after the output is finished

The wait-and-wake mechanism is needed:

Input: After assignment, execution method Wait () Waits forever

Output: After printing, before output waits, wake-up input notify (), and then wait () forever

Input: After being awakened, re-assigned, must notify () wake up the output of the thread, and then wait ()

Loop down in turn

Code implementation:

 Package demo;  Public class Resource {    public  String name;      Public String sex;      Public Boolean false ;}

 Packagedemo; Public classInputImplementsRunnable {PrivateResource R;  PublicInput (Resource r) { This. R =R; }     Public voidrun () {inti = 0;  while(true) {            synchronized(r) {if(r.flag) {Try{r.wait (); } Catch(Exception e) {}}if(i% 2 = = 0) {R.name= "Zhang San"; R.sex= "Male"; } Else{r.name= "John Doe"; R.sex= "female"; } R.flag=true;            R.notify (); } I++; }    }}

 Packagedemo; Public classOutputImplementsRunnable {PrivateResource R;  PublicOutput (Resource r) { This. R =R; }     Public voidrun () { while(true) {            synchronized(r) {if(!r.flag) {Try{r.wait (); } Catch(Exception e) {}} System.out.println (R.name+ "..." +r.sex); R.flag=false;            R.notify (); }        }    }}

 Packagedemo; Public classThreaddemo { Public Static voidMain (string[] args) {Resource R=NewResource (); Input in=NewInput (R); Output out=NewOutput (R); Thread Tin=NewThread (in); Thread Tout=NewThread (out);        Tin.start ();    Tout.start (); }}

This is the Dick and Harry interleaved output.

Complete

Java Learning Note 46 (multithreading three: communication between threads)