Statements in a transformer SelectionKey key socket.getIOChannel().keyFor( socket.getPoller().getSelector()); Assignment(42) Conditidnal (15) Loop (2) extraction V=E if(E){…3 for (x:E) g1=(1.getIOChannel().keyFor(2.getPoller().getSelector())) else {... {..3 E(42) E(15) E(2) if (name.startswith("selectorPool.")) extraction Source 8 Source g2 =(1.startswith(2)) Null-check 93 =("selectorPool.") 7 94 =(01.startswith("selectorPool.")) 12 Fig.3:Statistics of the basic constructs in the studied non-trivial while (paused &!running) object transformers. extraction 95=(o1&D2) Transformer Fig.5:Examples of extracted gadgets. Statement v e; obj.f=e; if (c){s*else s*) while (c){s*} A.The Language Design Condition = ee =null!c Following the empirical findings that default transformations Expression vIg(v) and existing code snippets are the major constructs of a non- Gadget 9= extracted gadgets trivial object transformer,we made the following choices in Variable 0 = stale v v2... the DSL design: 1)Providing a mechanism for code reuse.Particularly,we Fig.4:Syntax of basic constructs in object transformers.f is provide a DSL construct named gadget,as denoted by a field subject to transformation;stale and obj are the stale g(),a textural expression extracted from source code object and its corresponding new-version object;a*denotes with all variable references being replaced by a place- zero or more repeats of a. holder.We use angled brackets to enclose a gadget,e.g., g(o1,☐2，☐3)=(O1.foo(o2,☐3)）.Applying a gadget to an object transformer would reuse the entire expression synthesis mechanism.As shown in Figure 3,the vast majority with the flexibility for placeholders to be filled with (54/59,or 91.5%)of the basic constructs are either: transformer-specific values. 1)a trivial default behavior (13/59,or 22.0%), 2)Providing no arithmetic,logical,or bitwise operator.We 2)a single member method call (12/59,or 20.3%), argue that when such operators (+,&&1,...)should 3)a simple null-check (7/59,or 11.9%),or appear in an object transformer,they will also likely to 4)a minor revision of an existing source code snippet (22/59, exist in old/new version source code and can be extracted or 37.3%)like Lines 23-24 in Figure 1. as gadgets.Therefore,we do not have to include them in Such a result motivated us to synthesize object transformers the DSL.yielding a minimal,concise DSL by assembling source code gadgets (extracted expressions from 3)Providing limited expressiveness for branch/loop condi- old/version source code)upon a domain-specific language tions.Branch/loop conditions in object transformers are designated for the object transformation in DSU. also likely in the existing source code.Therefore,nega- For the remaining a few (5/59,or 8.5%)basic constructs, tions,nested branches,and while-loops provide sufficient three of them are boolean configuration-related constants expressiveness for constructing object transformers. whose values are determined by an update-time configuration. Figure 4 lists the syntax of our DSL.An object transformer The other two expressions need a reference that is not reachable t is a sequence of statements s*,in which each field of the from stale objects.Since this paper focuses on the automatic new-version object obj is assigned with a value.For each synthesis of object transformers,we leave these relatively rare statement s,it can define a new variable v:by applying a cases to future work. gadget g and filling its placeholders with existing variables(a previously defined vi or stale),assign a field to be transformed IV.DOMAIN-SPECIFIC LANGUAGE FOR OBJECT (obj.f)with a value,or use if branches or while loops with TRANSFORMATION a condition c. This section explains our design goals and choices in Readers may notice that the statements in a transformer t our domain-specific language (DSL)for describing object describe a skeleton,which specifies the targeted transformer's transformers.The DSL design and gadget extraction are 5We allow a void-typed expression to be assigned to a variable,i.e..g can described in Sections IV-A and IV-B,respectively. be a void method invocation.Default Member 13 12 Source 12 Others 5 Source 8 Null-check 7 Source 2 v = E if (E) {...} else {...} for (x: E) {...} Statements in a transformer E (42) E (15) E (2) Assignment (42) Conditional (15) Loop (2) Fig. 3: Statistics of the basic constructs in the studied non-trivial object transformers. Transformer t ::= s ∗ Statement s ::= v = e; | obj.f = e; | if (c) { s ∗ } else { s ∗ } | while (c) { s ∗ } Condition c ::= e | e == null | !c Expression e ::= v | g(v ∗ ) Gadget g ::= extracted gadgets Variable v ::= stale | v1 | v2 | . . . Fig. 4: Syntax of basic constructs in object transformers. f is a field subject to transformation; stale and obj are the stale object and its corresponding new-version object; a ∗ denotes zero or more repeats of a. synthesis mechanism. As shown in Figure 3, the vast majority (54/59, or 91.5%) of the basic constructs are either: 1) a trivial default behavior (13/59, or 22.0%), 2) a single member method call (12/59, or 20.3%), 3) a simple null-check (7/59, or 11.9%), or 4) a minor revision of an existing source code snippet (22/59, or 37.3%) like Lines 23–24 in Figure 1. Such a result motivated us to synthesize object transformers by assembling source code gadgets (extracted expressions from old/version source code) upon a domain-specific language designated for the object transformation in DSU. For the remaining a few (5/59, or 8.5%) basic constructs, three of them are boolean configuration-related constants whose values are determined by an update-time configuration. The other two expressions need a reference that is not reachable from stale objects. Since this paper focuses on the automatic synthesis of object transformers, we leave these relatively rare cases to future work. IV. DOMAIN-SPECIFIC LANGUAGE FOR OBJECT TRANSFORMATION This section explains our design goals and choices in our domain-specific language (DSL) for describing object transformers. The DSL design and gadget extraction are described in Sections IV-A and IV-B, respectively. SelectionKey key = socket.getIOChannel().keyFor( socket.getPoller().getSelector()); g1 = h1.getIOChannel().keyFor(2.getPoller().getSelector())i extraction if (name.startsWith("selectorPool.")) g2 = h1.startsWith(2)i g3 = h"selectorPool."i g4 = h1.startsWith("selectorPool.")i extraction while (paused && !running) g5 = h1 && !2i extraction Fig. 5: Examples of extracted gadgets. A. The Language Design Following the empirical findings that default transformations and existing code snippets are the major constructs of a non￾trivial object transformer, we made the following choices in the DSL design: 1) Providing a mechanism for code reuse. Particularly, we provide a DSL construct named gadget, as denoted by g(~), a textural expression extracted from source code with all variable references being replaced by a place￾holder. We use angled brackets to enclose a gadget, e.g., g(1, 2, 3) = h1.foo(2,3)i. Applying a gadget to an object transformer would reuse the entire expression with the flexibility for placeholders to be filled with transformer-specific values. 2) Providing no arithmetic, logical, or bitwise operator. We argue that when such operators (+, &&, |, . . .) should appear in an object transformer, they will also likely to exist in old/new version source code and can be extracted as gadgets. Therefore, we do not have to include them in the DSL, yielding a minimal, concise DSL. 3) Providing limited expressiveness for branch/loop condi￾tions. Branch/loop conditions in object transformers are also likely in the existing source code. Therefore, nega￾tions, nested branches, and while-loops provide sufficient expressiveness for constructing object transformers. Figure 4 lists the syntax of our DSL. An object transformer t is a sequence of statements s ∗ , in which each field of the new-version object obj is assigned with a value. For each statement s, it can define a new variable vi by applying a gadget g 5 and filling its placeholders with existing variables (a previously defined vi or stale), assign a field to be transformed (obj.f) with a value, or use if branches or while loops with a condition c. Readers may notice that the statements in a transformer t describe a skeleton, which specifies the targeted transformer’s 5We allow a void-typed expression to be assigned to a variable, i.e., g can be a void method invocation.