Unintentional thread synchronization when writing to file

Question

If I have a tread that counts to 1000 and prints out the result like so:

public class MyRunnable implements Runnable{
    @Override
    public  void run() {
        for (int i=0;i<1000;i++){
            System.out.println(i+" ");
        }
    }
}

And in main I do something like:

new Thread(new MyRunnable()).start();
new Thread(new MyRunnable()).start();

the results get jumbled up, since I'm not synchronizing.

But, if my thread looks like:

public class MyRunnable implements Runnable{
    Writer w;
    MyRunnable(Writer w){
        this.w=w;
    }

    @Override
    public  void run() {
        for (int i=0;i<1000;i++){
            try{
                w.write(String.valueOf(i));
                w.write (" ") ;
            }//&catch the exception
        }
    }
}

and my in main I have this this:

  try (Writer writer = new BufferedWriter(new OutputStreamWriter(
                    new FileOutputStream("Thread.txt"), "utf-8"))) {
       Thread t1 =new Thread(new MyRunnable(writer));
       Thread t2 =new Thread(new MyRunnable(writer));
       t1.start(); 
       t2.start();
       t1.join();
       t2.join();
   }    //catch the exception,etc.

The results look normal: the threads somehow got synchronized. I imagine it's because I pass writer as a parameter, but could someone explain it better? And, out of curiosity, is there any way around it?

EDIT: My apologies, it seems i hurried too much. I tried to simplify MyThread and removed too much code. but i just realized that the situation i described only happens if i try to open and read a file in run() method. So run actually looks like this:

public  void run() {
    try {
                Scanner scanner = new Scanner(f); //f is new File (filename) 
                while (scanner.hasNext())
                {
                    String str = scanner.next();
                    if (str.equals("word")) //code counts "word" appearance
                        count++;
                }
                scanner.close();
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            } 
             for (int i=0;i<1000;i++){
                try{
                    w.write(String.valueOf(i));
                    w.write (" ") ;
                }//&catch the exception
            } //continue by writing the count

I suppose this means the problem is actually in reading the file?

Answer 1

Writer的write(String)方法（ BufferedWriter从中扩展）是同步的。

Answer 2

This one will produce output in a non-synchronized manner. It is similar to yours, so I think there is something you are omitting.

import java.io.BufferedWriter;
import java.io.IOException;
import java.io.Writer;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;

/**
 */
public class ThreadTest {
    final static int N = 1000;

    static Runnable countDown(Writer writer){
        return ()->{
            for(int i = 0; i<N; i++){
                try{
                    writer.write(String.format("%d\t", i));
                } catch(Exception exc){

                }
            }

            try {
                writer.write("\n");
            } catch (IOException e) {
                e.printStackTrace();
            }
        };
    }

    public static void main(String[] args){
        try(
            Writer writer = Files.newBufferedWriter(Paths.get("test.txt"), StandardCharsets.UTF_8, StandardOpenOption.CREATE, StandardOpenOption.TRUNCATE_EXISTING)
        ){
            Thread a = new Thread(countDown(writer));
            Thread b = new Thread(countDown(writer));
            a.start();
            b.start();
            a.join();
            b.join();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }

}

Then the output becomes:

0   0   1   1   2   2   3   3   4   4   5   6   5   6   7   7   8   8   9   10  9   10  11  11  12  13  12  13  14  14  15  15  16  16  17  17  18  18  19  19  20  20  21  21  22  22  23  23  24  24  25  25  26  26  27  27  28  28  29  29  30  30  31  32  31  33  32  34  33  35  34  36  37  35  38  36  39  37  40  38  41  39  42  40  43  41  42  44  43  45  44  46  45  47  46  48  47  49  50  48  49  51  50  52  53  51  54  52  55  56  53  57  54  58  55  59  56  60  57  61  58  62  59  63  60  64  61  62  65  63  66  64  67  65  68  66  69  67  70  68  71  69  72  70  73  71  74  72  75  73  76  74  77  75  78  76  77  79  78  80  79  81  80  82  81  83  84  82  85  83  86  84  87  85  88  86  89  87  90  88  91  89  92  90  93  91  94  92  95  93  96  97  94  98  95  99  96  100 97  101 98  102 99  103 100 104 101 105 102 106 103 107 104 108 105 109 106 110 107 111 108 112 109 113 110 114 111 115 112 116 113 117 114 118 115 119 116 120 117 121 118 122 119 123 120 124 121 125 122 126 123 127 124 128 125 129 126 130 127 131 128 132 129 133 130 134 131 135 132 136 133 137 134 138 135 139 136 140 137 141 138 142 139 143 140 144 141 145 142 146 143 147 144 148 145 149 146 150 147 151 148 152 149 153 150 154 151 155 152 156 153 157 154 158 155 159 156 160 157 158 161 162 159 163 160 164 161 165 162 166 163 167 164 168 165 169 166 170 167 171 168 172 169 173 170 174 171 175 172 176 173 177 174 178 175 179 176 180 177 181 178 182 179 183 180 184 181 185 182 186 183 187 184 188 185 189 186 190 187 191 188 192 189 193 190 191 194 192 195 193 196 194 197 195 198 196 199 197 200 198 201 199 202 203 200 204 201 205 202 206 203 207 204 208 209 205 210 206 211 207 212 213 208 214 209 215 210 216 211 217 212 218 213 219 214 220 215 221 216 222 217 223 218 224 219 225 220 226 221 227 222 228 223 229 224 230 225 231 226 232 227 233 234 228 235 236 229 237 230 238 231 239 232 233 240 234 241 235 242 236 243 237 244 238 245 239 246 247 240 241 248 242 249 243 250 244 251 245 252 253 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815 685 816 817 686 818 687 819 820 688 821 689 822 823 690 824 691 825 826 692 827 693 828 829 694 830 695 831 832 696 833 697 834 835 698 836 699 837 838 700 839 701 840 841 702 842 703 843 704 844 705 845 706 846 707 847 708 848 709 849 710 850 711 851 712 852 853 713 854 714 855 715 856 716 857 717 858 718 859 719 860 720 861 721 862 722 863 723 864 724 865 725 866 726 867 727 868 728 869 729 870 730 871 731 872 732 873 733 874 734 875 735 876 736 877 737 878 738 879 739 880 740 881 741 882 742 883 743 884 744 885 745 886 746 887 747 748 888 889 749 890 891 750 892 893 751 894 895 752 896 753 897 898 754 899 900 755 901 902 756 903 757 904 905 758 906 907 759 908 760 909 910 761 911 912 762 913 914 763 915 916 764 917 765 918 919 766 920 921 767 922 923 768 924 769 925 926 770 927 928 771 929 772 930 931 773 932 774 933 934 775 935 936 776 937 777 938 939 778 940 779 941 780 942 943 781 944 945 782 946 783 947 948 784 949 785 950 951 786 952 787 953 954 788 955 789 956 957 790 958 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824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 

Which, as you can see ends up jumbled.

In the case of your Scanner, since you are making two Scanner for the same file on windows you are essentially synchronizing on the file. eg. The read/write condition ends up looking like this:

try{
    synchronized(f){
        Scanner s = new Scanner(f);
        System.out.println("starting");
        while(s.hasNext()){
            System.out.println(s.next().length());
        }
        System.out.println("finished");
        s.close();
    }
}catch(Exception e){e.printStackTrace();}

The result of this is that, one thread reads the file, finishes and starts counting, at that point the other thread starts reading the file. Reading the file takes much longer than counting down.

Answer 3

It's not clear what you want exactly. But if you just want t1 and t2 to mix up their numbers you should make sure they start exactly at the same time. An easy way to do that is to use a CountdownLatch that you store in your threads like this:

public class MyRunnable implements Runnable{
        Writer w;
        CountDownLatch countDownLatch;
        MyRunnable(Writer w, CountDownLatch countDownLatch){
            this.w=w;
            this.countDownLatch=countDownLatch;
        }

        @Override
        public  void run() {
            countDownLatch.await();
            for (int i=0;i<1000;i++){
                try{
                    w.write(String.valueOf(i));
                    w.write (" ") ;
                }//&catch the exception
            }
        }
}

And that you create before the thread and launch when they are created:

try (Writer writer = new BufferedWriter(new OutputStreamWriter(
        new FileOutputStream("Thread.txt"), "utf-8"))) {
    CountDownLatch countDownLatch = new CountDownLatch(1);
    Thread t1 =new Thread(new MyRunnable(writer, countDownLatch));
    Thread t2 =new Thread(new MyRunnable(writer), countDownLatch);
    t1.start();
    t2.start();
    countDownLatch.countDown(); //Start the threads
    t1.join();
    t2.join();
}    //catch the exception,etc.

This will make sure that the time taken for the thread creation doesn't influence your test and that both threads actually start at the same time.