Switching to StringBuilder

Author: ajermakovics

README.MD

@@ -3,5 +3,53 @@ Lambda2sql
 
 Convert Java 8 lambdas to SQL statements.
 
+For example, the following Predicate:
+```
+ person -> person.getAge() < 100 && person.getHeight() > 200
+```
+ 
+is converted to a string:
+ 
+```
+  age < 100 AND height > 200
+```
+allowing you to write readable queries in a type safe way.
 
+See Lambda2SqlTest for more examples.
+	
+Usage
+---------
+
+```
+Lambda2Sql.init(); // initialize on program start
+
+Predicate<Person> predicate = person -> person.getAge() < 100 && person.getHeight() > 200;
+
+String sql = Lambda2Sql.toSql( predicate ); // age < 100 AND height > 200
+```
+
+
+How it works
+---------
+
+It uses [JaQue](https://github.com/TrigerSoft/jaque) to build an expression tree for a lambda. The expression tree is then traversed and converted to a SQL statement. 
+
+Under the hood JaQue depends on the following system property:
+`jdk.internal.lambda.dumpProxyClasses`
+See [https://bugs.openjdk.java.net/browse/JDK-8023524](https://bugs.openjdk.java.net/browse/JDK-8023524).
+
+When the property is enabled, JVM generated classes for lambdas are saved to disk. JaQue then uses [ASM](http://asm.ow2.org/) to read the .class files and creates expression trees.
+
+
+Limitations
+---------
+
+Current version only works with predicates and supports the following operators: >, >=, <, <=, =, !=, &&, ||, !
+
+Build
+---------
+
+`gradle jar`
+
+or you can just [download the jar](http://dl.bintray.com/ajermakovics/jar/lambda2sql-0.1.jar).
 

build.gradle

@@ -5,6 +5,10 @@ sourceCompatibility = 1.8
 targetCompatibility = 1.8
 
 version = '0.1'
+group = "lambda2sql"
+archivesBaseName = "lambda2sql"
+jar.baseName = archivesBaseName
+
 jar {
     manifest {
         attributes 'Implementation-Title': 'Lambda2sql', 'Implementation-Version': version

src/main/java/lambda2sql/Lambda2Sql.java

@@ -37,6 +37,6 @@ public static void init() {
 	 */
 	public static <T> String toSql(Predicate<T> predicate) {
 		LambdaExpression<Predicate<T>> lambdaExpression = LambdaExpression.parse(predicate);
-		return lambdaExpression.accept(new ToSqlVisitor());
+		return lambdaExpression.accept(new ToSqlVisitor()).toString();
 	}
 }

src/main/java/lambda2sql/ToSqlVisitor.java

@@ -6,27 +6,24 @@
 
 import com.trigersoft.jaque.expression.*;
 
-public class ToSqlVisitor implements ExpressionVisitor<String> {
-
-	boolean top = true;
+public class ToSqlVisitor implements ExpressionVisitor<StringBuilder> {
 
+	private StringBuilder sb = new StringBuilder();
+	private Expression body;
 
 	@Override
-	public String visit(BinaryExpression e) {
-		boolean quote = !top && e.getExpressionType() == LogicalOr;
-		top = false;
+	public StringBuilder visit(BinaryExpression e) {
+		boolean quote = e != body && e.getExpressionType() == LogicalOr;
 
-		StringBuilder sb = new StringBuilder();
 		if( quote ) sb.append('(');
 
-		String first = e.getFirst().accept(this);
-		String second = e.getSecond().accept(this);
-
-		sb.append(first).append(' ').append(toSqlOp(e.getExpressionType())).append(' ').append(second);
+		e.getFirst().accept(this);
+		sb.append(' ').append(toSqlOp(e.getExpressionType())).append(' ');
+		e.getSecond().accept(this);
 
 		if( quote ) sb.append(')');
 
-		return sb.toString();
+		return sb;
 	}
 
 	public static String toSqlOp(int expressionType) {
@@ -39,35 +36,37 @@ public static String toSqlOp(int expressionType) {
 	}
 
 	@Override
-	public String visit(ConstantExpression e) {
-		return e.getValue().toString();
+	public StringBuilder visit(ConstantExpression e) {
+		return sb.append(e.getValue().toString());
 	}
 
 	@Override
-	public String visit(InvocationExpression e) {
+	public StringBuilder visit(InvocationExpression e) {
 		return e.getTarget().accept(this);
 	}
 
 	@Override
-	public String visit(LambdaExpression<?> e) {
-		return e.getBody().accept(this);
+	public StringBuilder visit(LambdaExpression<?> e) {
+		this.body = e.getBody();
+		return body.accept(this);
 	}
 
 	@Override
-	public String visit(MemberExpression e) {
+	public StringBuilder visit(MemberExpression e) {
 		String name = e.getMember().getName();
-		return name.replaceAll("^(get|is)", "").toLowerCase();
+		name = name.replaceAll("^(get|is)", "").toLowerCase();
+		return sb.append(name);
 	}
 
 	@Override
-	public String visit(ParameterExpression e) {
-		return "";
+	public StringBuilder visit(ParameterExpression e) {
+		return sb;
 	}
 
 	@Override
-	public String visit(UnaryExpression e) {
-		String first = e.getFirst().accept(this);
-		return ExpressionType.toString(e.getExpressionType()) + first;
+	public StringBuilder visit(UnaryExpression e) {
+		sb.append(ExpressionType.toString(e.getExpressionType()));
+		return e.getFirst().accept(this);
 	}
 
 }

src/test/java/lambda2sql/Lambda2SqlTest.java

@@ -33,8 +33,9 @@ public void testLogicalOps() throws Exception {
 	}
 
 	@Test
-	public void testMultiLogicalOps() throws Exception {
+	public void testMultipleLogicalOps() throws Exception {
 		assertEqual("active AND (age < 100 OR height > 200)", e -> e.isActive() && (e.getAge() < 100 || e.getHeight() > 200) );
+		assertEqual("(age < 100 OR height > 200) AND active", e -> (e.getAge() < 100 || e.getHeight() > 200) && e.isActive() );
 	}
 
 	private void assertEqual(String expectedSql, Predicate<Person> p) {