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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11To debug a Java Stream pipeline in IntelliJ IDEA, enable the Java Stream Debugger plugin, pause execution at a stream operation, then choose More > Trace Current Stream Chain in the Debug tool window. Use Split Mode to inspect operations one at a time or Flat Mode to see the pipeline’s transformations together. The trace helps show where values change; IntelliJ’s regular debugger tools help you inspect the exact predicate, mapper, or variable behind a wrong result.
How do I debug a Java Stream pipeline in IntelliJ IDEA?
- Enable the plugin. Open Settings on Windows or Linux, or Preferences on macOS, and go to Plugins. Install or enable Java Stream Debugger.
- Set a breakpoint. In project source, place a breakpoint on a line containing a stream operation, including the terminal operation.
- Start a debug session. Run the program with the debugger and let execution suspend at the breakpoint.
- Open the trace. In the Debug tool window, choose More > Trace Current Stream Chain. IntelliJ evaluates the current stream and opens the Stream Trace dialog.
- Inspect the pipeline. Choose Split Mode or Flat Mode, then examine how values change across the operations.
JetBrains describes the feature as a way to “visualize what is going on in a Java Stream.” The visualization follows data from the source through intermediate operations to the terminal operation, which can make an incorrect filter or mapper easier to locate.
How do I see what happens after each map or filter?
Split Mode: inspect operations individually
Split Mode presents operations separately, with tabs for moving between them. Use it when you want to focus on a particular stage and see how values look as they pass through the pipeline.
Flat Mode: view transformations together
Flat Mode shows the transformations together in one view, making the pipeline easier to scan as a whole. It does not display terminal operations that have no return value, such as forEach.
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What can Stream Trace show—and where does it stop?
- It needs a suspended debug session. The program must be paused at a stream operation for tracing to be useful or available.
- It traces project source, not library or decompiled code. A pipeline defined only in library code or decompiled code is outside the feature’s supported source boundary.
- It ends at the terminal operation. If the terminal operation returns a value that is then processed by another API, that later processing is not shown in the Stream Trace dialog. For example, a subsequent
Optionalstage is beyond the trace boundary. - Some newer operations may be version-sensitive. JetBrains YouTrack has an issue reporting that the debugger ignores
Stream.gather(), so it cannot detect, trace, or visualize that operation. Check the issue and your IDE build when working with newer Stream APIs: IDEA-364127.
Why is “Trace Current Stream Chain” disabled?
First check that the Java Stream Debugger plugin is installed and enabled. Then confirm that you started the program with the debugger and that execution is currently suspended at a line containing a stream operation. The action is intended for a live, paused stream chain; it is not a general command for inspecting an unstarted program or arbitrary source code.
When should I use ordinary debugger tools?
Stream Trace is useful for locating the stage where data becomes incorrect. Once you have narrowed the problem to a filter, mapper, or predicate, use IntelliJ’s normal debugger to inspect the code and state behind that transformation:
- Inspect variables, including values captured by a lambda.
- Add watches for expressions you want to monitor.
- Step through relevant code.
- Evaluate an expression while execution is suspended.
For example, if a trace shows that an unexpected value remains after a filter, inspect the predicate and its inputs with a watch or expression evaluation. The trace identifies where to investigate; ordinary debugging tools help explain why the operation produced that result. See JetBrains’ IntelliJ IDEA debugger documentation for the standard debugger workflow.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Stream Trace vs. step-by-step debugging
| Question | Stream Trace | Conventional debugger |
|---|---|---|
| Setup | Requires the Java Stream Debugger plugin, a breakpoint on a stream operation, and a suspended debug session. | Uses standard debugger features such as breakpoints and stepping; JetBrains documents these in its debugger guide. |
| Intermediate operations | Visualizes transformations across the stream chain in Split Mode or Flat Mode. | Does not provide the Stream Trace pipeline visualization; inspect execution by stepping through code and examining state. |
| Element-by-element transformation | Shows how stream values change at pipeline stages, subject to the trace boundaries and supported operations. | Use variable inspection, watches, and expression evaluation to investigate particular values and code. |
| Source coverage | Works with project files, not libraries or decompiled code. | Can inspect suspended program state with ordinary debugger tools; Stream Trace’s project-source restriction does not apply to those tools. |
| Terminal operations | Stops at the terminal operation; Flat Mode omits void terminal operations such as forEach. |
Step through the surrounding code and inspect state as needed. |
The two approaches complement each other: use Stream Trace to locate the transformation stage, then use watches, stepping, or expression evaluation to examine the specific logic and state.
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