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Showing content from https://binarydoc.blogspot.com/2019/10/what-is-java-synthetic-class.html below:

What is Java Synthetic Class (ACC_SYNTHETIC)? with the example of BoundMethodHandle$1.class in Java package java.lang.invoke

When a

file marked with the

flag, it indicates the

file was generated by

and does not appear in the corresponding

.

Binary Code is always the best document, so let's check an example: the BoundMethodHandle$1.class class in java.base.jmod in JDK.

The class file BoundMethodHandle$1.class has been marked as synthetic flag, as shown bellow:

And this class is used by the container class

:

This class does not exist in the original source code

, but generated by the java compiler, from the following

methods in the Java class

:

1. BoundMethodHandle.bindSingle(), at byte code offset 0
2. BoundMethodHandle.arg(), at byte code offset 17

Highlight: Source code of

:

62  static BoundMethodHandle bindSingle(MethodType type, LambdaForm form, BasicType xtype, Object x) {
63      // for some type signatures, there exist pre-defined concrete BMH classes
64      try {
65          switch (xtype) {
66          case L_TYPE:
67              return bindSingle(type, form, x);  // Use known fast path.
68          case I_TYPE:
69              return (BoundMethodHandle) SPECIALIZER.topSpecies().extendWith(I_TYPE_NUM).factory().invokeBasic(type, form, ValueConversions.widenSubword(x));
70          case J_TYPE:
71              return (BoundMethodHandle) SPECIALIZER.topSpecies().extendWith(J_TYPE_NUM).factory().invokeBasic(type, form, (long) x);
72          case F_TYPE:
73              return (BoundMethodHandle) SPECIALIZER.topSpecies().extendWith(F_TYPE_NUM).factory().invokeBasic(type, form, (float) x);
74          case D_TYPE:
75              return (BoundMethodHandle) SPECIALIZER.topSpecies().extendWith(D_TYPE_NUM).factory().invokeBasic(type, form, (double) x);
76          default : throw newInternalError("unexpected xtype: " + xtype);
77          }
78      } catch (Throwable t) {
79          throw uncaughtException(t);
80      }
81  }

Highlight: Source code of

:

178  /*non-public*/ final Object arg(int i) {
179      try {
180          Class fieldType = speciesData().fieldTypes().get(i);
181          switch (BasicType.basicType(fieldType)) {
182              case L_TYPE: return          speciesData().getter(i).invokeBasic(this);
183              case I_TYPE: return (int)    speciesData().getter(i).invokeBasic(this);
184              case J_TYPE: return (long)   speciesData().getter(i).invokeBasic(this);
185              case F_TYPE: return (float)  speciesData().getter(i).invokeBasic(this);
186              case D_TYPE: return (double) speciesData().getter(i).invokeBasic(this);
187          }
188      } catch (Throwable ex) {
189          throw uncaughtException(ex);
190      }
191      throw new InternalError("unexpected type: " + speciesData().key()+"."+i);
192  }

So we can found that, the key reason for the compiler to generate the

class, and its

field is, there is an

statment on the

value.

Here is the highlighted source code of the LambdaForm.BasicType:

139  enum BasicType {
140      L_TYPE('L', Object.class, Wrapper.OBJECT),  // all reference types
141      I_TYPE('I', int.class,    Wrapper.INT),
142      J_TYPE('J', long.class,   Wrapper.LONG),
143      F_TYPE('F', float.class,  Wrapper.FLOAT),
144      D_TYPE('D', double.class, Wrapper.DOUBLE),  // all primitive types
145      V_TYPE('V', void.class,   Wrapper.VOID);    // not valid in all contexts
...
155      static final byte
156              L_TYPE_NUM = (byte) L_TYPE.ordinal(),
157              I_TYPE_NUM = (byte) I_TYPE.ordinal(),
158              J_TYPE_NUM = (byte) J_TYPE.ordinal(),
159              F_TYPE_NUM = (byte) F_TYPE.ordinal(),
160              D_TYPE_NUM = (byte) D_TYPE.ordinal(),
161              V_TYPE_NUM = (byte) V_TYPE.ordinal();

And we can easily double confirm it is indeed the source code for the generated

logic of

.

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