13.7 Java Threads 603Next_In,
Next_Out : Integer range 1..Bufsize := 1;
end Buffer;protected body Buffer is
entry Deposit(Item : in Integer)
when Filled < Bufsize is
begin
Buf(Next_In) := Item;
Next_In := (Next_In mod Bufsize) + 1;
Filled := Filled + 1;
end Deposit;
entry Fetch(Item : out Integer) when Filled > 0 is
begin
Item := Buf(Next_Out);
Next_Out := (Next_Out mod Bufsize) + 1;
Filled := Filled - 1;
end Fetch;
end Buffer;13.6.7 Evaluation
Using the general message-passing model of concurrency to construct monitors
is like using Ada packages to support abstract data types—both are tools that
are more general than is necessary. Protected objects are a better way to provide
synchronized access to shared data.
In the absence of distributed processors with independent memories, the
choice between monitors and tasks with message passing as a means of imple-
menting synchronized access to shared data in a concurrent environment is
somewhat a matter of taste. However, in the case of Ada, protected objects are
clearly better than tasks for supporting concurrent access to shared data. Not
only is the code simpler; it is also much more efficient.
For distributed systems, message passing is a better model for concurrency,
because it naturally supports the concept of separate processes executing in
parallel on separate processors.13.7 Java Threads
The concurrent units in Java are methods named run, whose code can be in
concurrent execution with other such methods (of other objects) and with the
main method. The process in which the run methods execute is called a
thread. Java’s threads are lightweight tasks, which means that they all run in
the same address space. This is different from Ada tasks, which are heavyweight