Scala's lazy val is not thread safe. See line 65, I don't see anything related to synchronization.

lazy-val-javap.txt

~> scala
Welcome to Scala version 2.11.6 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_66).
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scala> class Foo { lazy val bar = 2 }
defined class Foo

scala> :javap Foo
  Size 831 bytes
  MD5 checksum 14727318592c9c4a86120db0d8e942af
  Compiled from "<console>"
public class Foo
  minor version: 0
  major version: 50
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Utf8               Foo
   #2 = Class              #1             // Foo
   #3 = Utf8               java/lang/Object
   #4 = Class              #3             // java/lang/Object
   #5 = Utf8               <console>
   #6 = Utf8               bar
   #7 = Utf8               I
   #8 = Utf8               bitmap$0
   #9 = Utf8               Z
  #10 = Utf8               bar$lzycompute
  #11 = Utf8               ()I
  #12 = NameAndType        #8:#9          // bitmap$0:Z
  #13 = Fieldref           #2.#12         // Foo.bitmap$0:Z
  #14 = NameAndType        #6:#7          // bar:I
  #15 = Fieldref           #2.#14         // Foo.bar:I
  #16 = Utf8               scala/runtime/BoxedUnit
  #17 = Class              #16            // scala/runtime/BoxedUnit
  #18 = Utf8               UNIT
  #19 = Utf8               Lscala/runtime/BoxedUnit;
  #20 = NameAndType        #18:#19        // UNIT:Lscala/runtime/BoxedUnit;
  #21 = Fieldref           #17.#20        // scala/runtime/BoxedUnit.UNIT:Lscala/runtime/BoxedUnit;
  #22 = Utf8               this
  #23 = Utf8               LFoo;
  #24 = Utf8               java/lang/Throwable
  #25 = Class              #24            // java/lang/Throwable
  #26 = NameAndType        #10:#11        // bar$lzycompute:()I
  #27 = Methodref          #2.#26         // Foo.bar$lzycompute:()I
  #28 = Utf8               <init>
  #29 = Utf8               ()V
  #30 = NameAndType        #28:#29        // "<init>":()V
  #31 = Methodref          #4.#30         // java/lang/Object."<init>":()V
  #32 = Utf8
  #33 = Class              #32            //
  #34 = Utf8               $line3/$read
  #35 = Class              #34            // $line3/$read
  #36 = Utf8
  #37 = Utf8
  #38 = Class              #37            //
  #39 = Utf8               Foo
  #40 = Utf8               Code
  #41 = Utf8               LocalVariableTable
  #42 = Utf8               LineNumberTable
  #43 = Utf8               StackMapTable
  #44 = Utf8               SourceFile
  #45 = Utf8               InnerClasses
  #46 = Utf8               Scala
{
  public int bar();
    descriptor: ()I
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: getfield      #13                 // Field bitmap$0:Z
         4: ifeq          14
         7: aload_0
         8: getfield      #15                 // Field bar:I
        11: goto          18
        14: aload_0
        15: invokespecial #27                 // Method bar$lzycompute:()I
        18: ireturn
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      19     0  this   LFoo;
      LineNumberTable:
        line 7: 0
      StackMapTable: number_of_entries = 2
        frame_type = 14 /* same */
        frame_type = 67 /* same_locals_1_stack_item */
          stack = [ int ]

  public Foo();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #31                 // Method java/lang/Object."<init>":()V
         4: return
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   LFoo;
      LineNumberTable:
        line 11: 0
}
SourceFile: "<console>"
InnerClasses:
     public static #36= #33 of #35; //=class  of class $line3/$read
     public static #36= #38 of #33; //=class  of class
     public static #39= #2 of #38; //Foo=class Foo of class
Error: unknown attribute
  Scala: length = 0x0

In line 77 it's calling lzyCompute. If you look at the body of that, I'm sure you'll find that it opens with a monitorenter.

class Something {
  lazy val foo = getFoo
  def getFoo = "foo!"
}

Using procyon-decompiler, this produces the following:

import scala.reflect.*;
import scala.runtime.*;

//
// Decompiled by Procyon v0.5.30
//

@ScalaSignature(bytes = "\u0006\u0001\u00012A!\u0001\u0002\u0001\u000b\tI1k\\7fi\"Lgn\u001a\u0006\u0002\u0007\u00059A(Z7qift4\u0001A\n\u0003\u0001\u0019\u0001\"a\u0002\u0006\u000e\u0003!Q\u0011!C\u0001\u0006g\u000e\fG.Y\u0005\u0003\u0017!\u0011a!\u00118z%\u00164\u0007\"B\u0007\u0001\t\u0003q\u0011A\u0002\u001fj]&$h\bF\u0001\u0010!\t\u0001\u0002!D\u0001\u0003\u0011!\u0011\u0002\u0001#b\u0001\n\u0003\u0019\u0012a\u00014p_V\tA\u0003\u0005\u0002\u001655\taC\u0003\u0002\u00181\u0005!A.\u00198h\u0015\u0005I\u0012\u0001\u00026bm\u0006L!a\u0007\f\u0003\rM#(/\u001b8h\u0011!i\u0002\u0001#A!B\u0013!\u0012\u0001\u00024p_\u0002BQa\b\u0001\u0005\u0002M\taaZ3u\r>|\u0007")
public class Something
{
    private String foo;
    private volatile boolean bitmap$0;

    private String foo$lzycompute() {
        synchronized (this) {
            if (!this.bitmap$0) {
                this.foo = this.getFoo();
                this.bitmap$0 = true;
            }
            final BoxedUnit unit = BoxedUnit.UNIT;
            return this.foo;
        }
    }

    public String foo() {
        return this.bitmap$0 ? this.foo : this.foo$lzycompute();
    }

    public String getFoo() {
        return "foo!";
    }
}