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This noncompliant code example involves the method sendPage() which that sends a Page object containing information being passed between a client and a server. The method is synchronized to protect access to the array pageBuff. Calling writeObject() within the synchronized sendPage can lead to result in a deadlock for high latency or lossy network connections.
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// Class Page is defined separately. It stores and returns the Page name via getName()
public final boolean SUCCESS = true;
public final boolean FAILURE = false;
Page[] pageBuff = new Page[MAX_PAGE_SIZE];
public synchronized boolean sendPage(Socket socket, String pageName){
try{
// Get the output stream to write the Page to
ObjectOutputStream out = new
ObjectOutputStream(socket.getOutputStream());
Page targetPage = null;
// Find the Page requested by the client
for(Page p : pageBuff){
if(p.getName().compareTo(pageName) == 0)
targetPage = p;
}
// Page requested does not exist
if(targetPage == null)
return FAILURE;
// Send the Page to the client
out.writeObject(targetPage);
out.flush();
out.close();
} catch(IOException io){ /* forward handleto exceptionhandler */ }
return SUCCESS;
}
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Compliant Solution
One This compliant solution entails separating the actions into a sequence of steps:
- Perform actions on data structures requiring synchronization
- Create copies of Objects objects to send
- Perform network calls in a separate method that does not require any synchronization
In the following example, the synchronized method getPage() is called from SendReply() to find the appropriate Page requested by the client from the Page array pageBuff of type Page. The method sendReply() in turn calls the unsynchronized method sendPage() to deliver the Page.
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public boolean sendReply(Socket socket, String pageName){
Page targetPage = getPage(pageName);
if(targetPage == null)
return FAILURE;
return sendPage(socket, targetPage);
}
private synchronized Page getPage(String pageName){
Page targetPage = null;
for(Page p : pageBuff){
if(p.getName().equals(pageName))
targetPage = p;
}
return targetPage;
}
public boolean sendPage(Socket socket, Page page){
try{
// Get the output stream to write the Page to
ObjectOutputStream out = new
ObjectOutputStream(socket.getOutputStream());
// Send the Page to the client
out.writeObject(page);
out.flush();
out.close();
return SUCCESS;
}catch(IOException io){
// handle exception else return failure
}
return FAILURE;
}
}
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Risk Assessment
If monitor regions such as synchronized methods and statements contain network transactional logic, temporary or permanent deadlocks may result.
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