Rollback of ThreadLocal optimization change

java/com/google/re2j/Pattern.java

@@ -58,7 +58,7 @@ public final class Pattern implements Serializable {
   }
 
   /**
-   * Releases current thread memory used by internal caches associated with this pattern. Does
+   * Releases memory used by internal caches associated with this pattern. Does
    * not change the observable behaviour. Useful for tests that detect memory
    * leaks via allocation tracking.
    */

java/com/google/re2j/RE2.java

@@ -110,8 +110,8 @@ class RE2 {
   int prefixRune;               // first rune in prefix
 
   // Cache of machines for running regexp.
-  // Warning: Don't use initialValue, GWT doesn't have that method.
-  private final ThreadLocal<Machine> machineThreadLocal = new ThreadLocal<Machine>();
+  // Accesses must be serialized using |this| monitor.
+  private final List<Machine> machine = new ArrayList<Machine>();
 
   // This is visible for testing.
   RE2(String expr) {
@@ -199,11 +199,6 @@ static RE2 compileImpl(String expr, int mode, boolean longest)
     return re2;
   }
 
-  // Clears the memory associated with this machine for the current thread.
-  void reset() {
-    machineThreadLocal.remove();
-  }
-
   /**
    * Returns the number of parenthesized subexpressions in this regular
    * expression.
@@ -212,6 +207,29 @@ int numberOfCapturingGroups() {
     return numSubexp;
   }
 
+  // get() returns a machine to use for matching |this|.  It uses |this|'s
+  // machine cache if possible, to avoid unnecessary allocation.
+  synchronized Machine get() {
+    int n = machine.size();
+    if (n > 0) {
+      return machine.remove(n - 1);
+    }
+    return new Machine(this);
+  }
+
+  // Clears the memory associated with this machine.
+  synchronized void reset() {
+    machine.clear();
+  }
+
+  // put() returns a machine to |this|'s machine cache.  There is no attempt to
+  // limit the size of the cache, so it will grow to the maximum number of
+  // simultaneous matches run using |this|.  (The cache empties when |this|
+  // gets garbage collected.)
+  synchronized void put(Machine m) {
+    machine.add(m);
+  }
+
   @Override
   public String toString() {
     return expr;
@@ -221,13 +239,11 @@ public String toString() {
   // the position of its subexpressions.
   // Derived from exec.go.
   private int[] doExecute(MachineInput in, int pos, int anchor, int ncap) {
-    Machine m = machineThreadLocal.get();
-    if (m == null) {
-      m = new Machine(this);
-      machineThreadLocal.set(m);
-    }
+    Machine m = get();
     m.init(ncap);
-    return m.match(in, pos, anchor) ? m.submatches() : null;
+    int[] cap = m.match(in, pos, anchor) ? m.submatches() : null;
+    put(m);
+    return cap;
   }
 
   /**