synchronized methods to execute during unsynchronized sections, improving performance and in Java

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synchronized methods to execute during unsynchronized sections, improving performance and
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reducing lock interference However, synchronization can be split only when the different parts of the method are not in any way dependent, so that it is acceptable for other methods to "see" and use the object before full method completion To illustrate, consider the following generic server class If helperoperation takes a long time, then calls to synchronized methods such as getState might block for an unacceptable time waiting for the method to be available
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class ServerWithStateUpdate { protected double state; protected final Helper helper = new Helper(); public synchronized void service() { state = ; // set to some new value helperoperation(); } public synchronized double getState() { return state; } }
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If helper represents some aspect of the host's state, or the call to helperoperation relies on or modifies host state, then the entire service method must employ synchronization However, if helperoperation is otherwise independent of the host, then the service method can be structured in the form suggested by the default rules in 1111:
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First, update state (holding locks) Then, send messages (without holding locks)
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Messages sent without holding locks are also known as open calls As discussed in 41 and 42, classes with methods of this form are among the most well-behaved and readily composable components in concurrent and event-based systems For example, assuming that helperoperation meets our criteria, the above class can be rewritten as:
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class ServerWithOpenCall { protected double state; protected final Helper helper = new Helper(); protected synchronized void updateState() { state = ; // set to some new value } public void service() { updateState(); helperoperation(); } }
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It is still possible to use open calls here even if the helper reference field is mutable; for example, using block synchronization:
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class ServerWithAssignableHelper { protected double state; protected Helper helper = new Helper(); synchronized void setHelper(Helper h) { helper = h; } public void service() { Helper h;
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synchronized(this) { state = h = helper; } hoperation(); } public synchronized void synchedService() { // see below service(); } }
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The synchedService method here reveals a weakness in any technique involving open calls The call to service from within synchedService results in the lock being held throughout the duration of service, including the call to hoperation This defeats the purpose of the method restructurings Avoiding such problems requires documentation of the intentional lack of synchronization in classes used in concurrent settings Data structures that are linked via immutable references are often amenable to these kinds of manipulations For example, consider a LinkedCell class in which each cell contains a reference to a successor cell, and for which we require that successor cell references be fixed upon construction This is a common requirement for cells serving as Lisp-style lists Methods and sections of methods solely involving the successor need not be synchronized, which speeds up traversal For clarity and emphasis, the methods here use recursion; in practice you would probably use iteration instead:
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class LinkedCell { protected int value; protected final LinkedCell next; public LinkedCell(int v, LinkedCell t) { value = v; next = t; } public synchronized int value() { return value; } public synchronized void setValue(int v) { value = v; } public int sum() { // add up all element values return (next == null) value() : value() + nextsum(); } public boolean includes(int x) { // search for x return (value() == x) true: (next == null) false : nextincludes(x); } }
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Again note that an object remains locked when a synchronized method calls an unsynchronized one So it would not avoid synchronization to write sum as:
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synchronized int ineffectivelyUnsynchedSum() { // bad idea return value + nextSum(); // synch still held on call } int nextSum() { return (next == null) 0: nextsum(); }
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