[performance] Avoid reading SourceFile twice

During compile parsing happens in two stages:
1. diet parse (any blocks like method bodies are skipped)
2. parse bodies
Both phases did read the source .java file from file system. With this
change the file contents is kept in CompilationResult.contentRef until
no longer needed. It is cached in
a SoftReference to avoid OutOfMemoryError.

eclipse-jdt#2691

org.eclipse.jdt.core.compiler.batch/src/org/eclipse/jdt/internal/compiler/CompilationResult.java

@@ -14,6 +14,7 @@
  *******************************************************************************/
 package org.eclipse.jdt.internal.compiler;
 
+import java.lang.ref.SoftReference;
 import java.util.ArrayList;
 /**
  * A compilation result consists of all information returned by the compiler for
@@ -87,6 +88,8 @@ public class CompilationResult {
 	private boolean hasMandatoryErrors;
 	public List<AnnotationBinding[]> annotations = new ArrayList<>(1);
 	private List<Runnable> scheduledProblems;
+	private volatile boolean cacheSource;
+	private volatile SoftReference<char[]> contentRef;
 
 	private static final int[] EMPTY_LINE_ENDS = Util.EMPTY_INT_ARRAY;
 	private static final Comparator PROBLEM_COMPARATOR = new Comparator() {
@@ -491,4 +494,29 @@ public void materializeProblems() {
 		}
 	}
 }
+
+public void cacheSource() {
+	this.cacheSource = true;
+}
+
+public char[] getContents() {
+	SoftReference<char[]> cr = this.contentRef;
+	if (cr != null) {
+		char[] cachedContents = cr.get();
+		if (cachedContents != null) {
+			return cachedContents;
+		}
+	}
+	return this.compilationUnit.getContents();
+}
+
+public void cacheContents(char[] contents) {
+	if (this.cacheSource) {
+		this.contentRef = new SoftReference<>(contents);
+	}
+}
+
+public void releaseContent() {
+	this.contentRef = null;
+}
 }

org.eclipse.jdt.core.compiler.batch/src/org/eclipse/jdt/internal/compiler/Compiler.java

@@ -869,6 +869,7 @@ protected void internalBeginToCompile(ICompilationUnit[] sourceUnits, int maxUni
 					if (this.totalUnits < this.parseThreshold) {
 						parsedUnit = this.parser.parse(sourceUnits[i], unitResult);
 					} else {
+						unitResult.cacheSource();
 						parsedUnit = this.parser.dietParse(sourceUnits[i], unitResult);
 					}
 					long resolveStart = System.currentTimeMillis();

org.eclipse.jdt.core.compiler.batch/src/org/eclipse/jdt/internal/compiler/ast/CompilationUnitDeclaration.java

@@ -186,6 +186,7 @@ public void cleanUp() {
 
 	if (this.scope != null)
 		this.scope.cleanUpInferenceContexts();
+	this.compilationResult.releaseContent();
 }
 
 private void cleanUp(TypeDeclaration type) {

org.eclipse.jdt.core.compiler.batch/src/org/eclipse/jdt/internal/compiler/parser/Parser.java

@@ -11615,7 +11615,7 @@ public void getMethodBodies(CompilationUnitDeclaration unit) {
 	CompilationResult compilationResult = unit.compilationResult;
 	char[] contents = this.readManager != null
 		? this.readManager.getContents(compilationResult.compilationUnit)
-		: compilationResult.compilationUnit.getContents();
+		: compilationResult.getContents();
 	this.scanner.setSource(contents, compilationResult);
 
 	if (this.javadocParser != null && this.javadocParser.checkDocComment) {
@@ -12852,6 +12852,7 @@ public CompilationUnitDeclaration parse(
 			problemReporter().cannotReadSource(this.compilationUnit, abortException, this.options.verbose);
 			contents = CharOperation.NO_CHAR; // pretend empty from thereon
 		}
+		compilationResult.cacheContents(contents);
 		this.scanner.setSource(contents);
 		this.compilationUnit.sourceEnd = this.scanner.source.length - 1;
 		if (end != -1) this.scanner.resetTo(start, end);