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POthreads accessing non-synchronised methods in Java
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  1. POthreads accessing non-synchronised methods in Java
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    copied!<p>can I ask to explain me how threads and synchronisation works in Java?</p> <p>I want to write a high-performance application. Inside this application, I read a data from files into some nested classes, which are basically a nut-shell around HashMap.</p> <p>After the data reading is finished, I start threads which need to go through the data and perform different checks on it. However, threads never change the data!</p> <p>If I can guarantee (or at least try to guarantee;) that my threads never change the data, can I use them calling non-synchronised methods of objects containing data?</p> <p>If multiple threads access the non-synchronised method, which does not change any class field, but has some internal variables, is it safe?</p> <p>artificial example:</p> <pre><code>public class Data{ // this hash map is filled before I start threads protected Map&lt;Integer, Spike&gt; allSpikes = new HashMap&lt;Integer, Spike&gt;(); public HashMap returnBigSpikes(){ Map&lt;Integer, Spike&gt; bigSpikes = new HashMap&lt;Integer, Spike&gt;(); for (Integer i: allSpikes.keySet()){ if (allSpikes.get(i).spikeSize &gt; 100){ bigSpikes.put(i,allSpikes.get(i)); } } return bigSpikes; } } </code></pre> <p>Is it safe to call a NON-synchronised method returnBigSpikes() from threads?</p> <p>I understand now that such use-cases are potentially very dangerous, because it's hard to control, that data (e.g., returned bigSpikes) will not be modified. But I have already implemented and tested it like this and want to know if I can use results of my application now, and change the architecture later...</p> <p>What happens if I make the methods synchronised? Will be the application slowed down to 1 CPU performance? If so, how can I design it correctly and keep the performance?</p> <p>(I read about 20-40 Gb of data (log messages) into the main memory and then run threads, which need to go through the all data to find some correlation in it; each thread becomes only a part of messages to analyse; but for the analysis, the thread should compare each message from its part with many other messages from data; that's why I first decided to allow threads to read data without synchronisation).</p> <p>Thank You very much in advance.</p>
 

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