SYNCHRONIZED

General Format

SYNCHRONIZED     [ ON Sync-Var ]       [Statement-1]     [Statement-2]     [Statement-n]     [END-SYNCHRONIZED]

Syntax Rules

1.	Statement-1 through statement-n represent either one or more imperative statements or a conditional statement optionally preceded by one or more imperative statements.

2.	Sync-Var is any item defined in the Data Division.

General rules

1.	SYNCHRONIZED statement allows to synchronize a block of statements when running in mutithread environment. When a thread is executing the statements of a synchronized block, other threads willing to execute the same block of statements block and wait for the first thread to finish.

2.	When the ON clause is specified, Sync-Var becomes the lock, otherwise the block itself is the lock. Two blocks that are synchronized on the same Sync-Var can’t be executed at the same time.

 

Examples

The following code may set the item varx to a unexpected value because it’s not safe to access the same data items from different threads:

working-storage section.  77 t1     handle of thread.  77 t2     handle of thread.  77 t3     handle of thread.  77 varx   pic XX.  procedure division.  main.     move "AA" to varx     perform thread p1 handle t1.     perform thread p2 handle t2.     perform thread p3 handle t3.     ACCEPT OMITTED     STOP RUN.   p1.            perform UNTIL 1 = 2        MOVE "AA" TO varx      end-perform.     p2.            perform UNTIL 1 = 2        MOVE "BB" TO varx      end-perform.     p3.            perform UNTIL 1 = 2        IF varx = "AA" or "BB"           continue        ELSE           display "Unexpected condition! varx=" varx           STOP RUN        END-IF      end-perform.

Synchronizing the access to varx as follows, resolves the issue:

working-storage section.  77 t1     handle of thread.  77 t2     handle of thread.  77 t3     handle of thread.  77 varx   pic XX.  procedure division.  main.     move "AA" to varx     perform thread p1 handle t1.     perform thread p2 handle t2.     perform thread p3 handle t3.     ACCEPT OMITTED     STOP RUN.   p1.            perform UNTIL 1 = 2        SYNCHRONIZED        MOVE "AA" TO varx        END-SYNCHRONIZED      end-perform.     p2.            perform UNTIL 1 = 2        SYNCHRONIZED        MOVE "BB" TO varx        END-SYNCHRONIZED      end-perform.     p3.            perform UNTIL 1 = 2        SYNCHRONIZED        IF varx = "AA" or "BB"           continue        ELSE           display "Unexpected condition! varx=" varx           STOP RUN        END-IF        END-SYNCHRONIZED      end-perform.

The following is a similar case that reproduces a NullPointerException instead of setting items to a undefined value:

working-storage section.  77 t1     handle of thread.  77 t2     handle of thread.  77 t3     handle of thread.  01 vars.     03 var1   pic 9(3).     03 var2   pic 9(5).  procedure division.  main.     move 1 to var1     move 2 to var2     perform thread P1 handle t1.     perform thread P2 handle t2.     perform thread P3 handle t3.     ACCEPT OMITTED     STOP RUN.   p1.            perform UNTIL 1 = 2        MOVE 1 TO var1      end-perform.     p2.            perform UNTIL 1 = 2        MOVE 2 TO var2      end-perform.     p3.            perform UNTIL 1 = 2        add 1 to var1        add 1 to var2      end-perform.

Also in this case, the SYNCHRONIZED statement allows to fix the issue:

working-storage section.  77 t1     handle of thread.  77 t2     handle of thread.  77 t3     handle of thread.  01 vars.     03 var1   pic 9(3).     03 var2   pic 9(5).  procedure division.  main.     move 1 to var1     move 2 to var2     perform thread P1 handle t1.     perform thread P2 handle t2.     perform thread P3 handle t3.     ACCEPT OMITTED     STOP RUN.   p1.            perform UNTIL 1 = 2        SYNCHRONIZED        MOVE 1 TO var1        END-SYNCHRONIZED      end-perform.     p2.            perform UNTIL 1 = 2        SYNCHRONIZED        MOVE 2 TO var2        END-SYNCHRONIZED      end-perform.     p3.            perform UNTIL 1 = 2        SYNCHRONIZED        add 1 to var1        add 1 to var2        END-SYNCHRONIZED      end-perform.