Weird monitorenter/monitorexit

example.clj

;; Clojure example:

(defn test-locks [this]
  (locking this
    2))

example.java

// Java example:
public class TestMonitors {

    public Object block(Integer a) {
        int b;
        synchronized (a) {
            b = new Integer(2);
        }
        b = a + b;
        return b;
    }
}

java_test.class

public class example.test.TestMonitors {
  public example.test.TestMonitors();
    Code:
       0: aload_0       
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return        

  public java.lang.Object block(java.lang.Integer);
    Code:
       0: aload_1       
       1: dup           
       2: astore_3      
       3: monitorenter  
       4: new           #2                  // class java/lang/Integer
       7: dup           
       8: iconst_2      
       9: invokespecial #3                  // Method java/lang/Integer."<init>":(I)V
      12: invokevirtual #4                  // Method java/lang/Integer.intValue:()I
      15: istore_2      
      16: aload_3       
      17: monitorexit   
      18: goto          28
      21: astore        4
      23: aload_3       
      24: monitorexit   
      25: aload         4
      27: athrow        
      28: aload_1       
      29: invokevirtual #4                  // Method java/lang/Integer.intValue:()I
      32: iload_2       
      33: iadd          
      34: istore_2      
      35: iload_2       
      36: invokestatic  #5                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
      39: areturn       
    Exception table:
       from    to  target type
           4    18    21   any
          21    25    21   any
}

test_locks.class

Compiled from "test.clj"
public final class example.test$test_locks extends clojure.lang.AFunction {
  public static final java.lang.Object const__0;

  public static {};
    Code:
       0: ldc2_w        #10                 // long 2l
       3: invokestatic  #17                 // Method java/lang/Long.valueOf:(J)Ljava/lang/Long;
       6: putstatic     #19                 // Field const__0:Ljava/lang/Object;
       9: return        

  public example.test$test_locks();
    Code:
       0: aload_0       
       1: invokespecial #22                 // Method clojure/lang/AFunction."<init>":()V
       4: return        

  public java.lang.Object invoke(java.lang.Object);
    Code:
       0: aload_1       
       1: aconst_null   
       2: astore_1      
       3: astore_2      
       4: aload_2       
       5: monitorenter  
       6: aconst_null   
       7: pop           
       8: getstatic     #19                 // Field const__0:Ljava/lang/Object;
      11: astore_3      
      12: aload_2       
      13: aconst_null   
      14: astore_2      
      15: monitorexit   
      16: aconst_null   
      17: pop           
      18: goto          32
      21: astore        4
      23: aload_2       
      24: aconst_null   
      25: astore_2      
      26: monitorexit   
      27: aconst_null   
      28: pop           
      29: aload         4
      31: athrow        
      32: aload_3       
      33: areturn       
    Exception table:
       from    to  target type
           4    12    21   any
}